Ketol-acid reductoisomerase using nadh

ABSTRACT

Methods for the evolution of NADPH specific ketol-add reductoisomerase enzymes to acquire NADH specificity are provided. Specific mutant ketol-acid reductoisomerase enzymes isolated from  Pseudomonas  that have undergone co-factor switching to utilize NADH are described.

This application is a continuation-in-part of U.S. Ser. No. 12/337,736,filed Dec. 18, 2008 and claims the benefit of the U.S. ProvisionalApplications, 61/015,346, filed Dec. 20, 2007, and 61/109,297, filedOct. 29, 2008.

FIELD OF THE INVENTION

The invention relates to protein evolution. Specifically, ketol-acidreductoisomerase enzymes have been evolved to use the cofactor NADHinstead of NADPH.

BACKGROUND OF THE INVENTION

Ketol-acid reductoisomerase enzymes are ubiquitous in nature and areinvolved in the production of valine and isoleucine, pathways that mayaffect the biological synthesis of isobutanol. Isobutanol isspecifically produced from catabolism of L-valine as a by-product ofyeast fermentation. It is a component of “fusel oil” that forms as aresult of incomplete metabolism of amino acids by yeasts. After theamine group of L-valine is harvested as a nitrogen source, the resultingα-keto acid is decarboxylated and reduced to isobutanol by enzymes ofthe Ehrlich pathway (Dickinson, et al., J. Biol. Chem., 273:25752-25756, 1998).

Addition of exogenous L-valine to the fermentation increases the yieldof isobutanol, as described by Dickinson et al., supra, wherein it isreported that a yield of isobutanol of 3 g/l is obtained by providingL-valine at a concentration of 20 g/L in the fermentation. In addition,production of n-propanol, isobutanol and isoamylalcohol has been shownby calcium alginate immobilized cells of Zymomonas mobilis (Oaxaca, etal., Acta Biotechnol., 11: 523-532, 1991).

An increase in the yield of C₃-C₅ alcohols from carbohydrates was shownwhen amino acids leucine, isoleucine, and/or valine were added to thegrowth medium as the nitrogen source (WO 2005040392).

While methods described above indicate the potential of isobutanolproduction via biological means these methods are cost prohibitive forindustrial scale isobutanol production. The biosynthesis of isobutanoldirectly from sugars would be economically viable and would represent anadvance in the art. However, to date the only ketol-acidreductoisomerase (KARI) enzymes known are those that bind NADPH in itsnative form, reducing the energy efficiency of the pathway. A KARI thatwould bind NADH would be beneficial and enhance the productivity of theisobutanol biosynthetic pathway by capitalizing on the NADH produced bythe existing glycolytic and other metabolic pathways in most commonlyused microbial cells. The discovery of a KARI enzyme that can use NADHas a cofactor as opposed to NADPH would be an advance in the art.

The evolution of enzymes having specificity for the NADH cofactor asopposed to NADPH is known for some enzymes and is commonly referred toas “cofactor switching”. See for example Eppink, et al. (J. Mol. Biol.,292: 87-96, 1999), describing the switching of the cofactor specificityof strictly NADPH-dependent p-Hydroxybenzoate hydroxylase (PHBH) fromPseudomonas fluorescens by site-directed mutagenesis; and Nakanishi, etal., (J. Biol. Chem., 272: 2218-2222, 1997), describing the use ofsite-directed mutagenesis on a mouse lung carbonyl reductase in whichThr-38 was replaced by Asp (T38D) resulting in an enzyme having a200-fold increase in the K_(M) values for NADP(H) and a correspondingdecrease of more than 7-fold in those for NAD(H). Co-factor switchinghas been applied to a variety of enzymes including monooxygenases,(Kamerbeek, et al., Eur. J. Biochem., 271: 2107-2116, 2004);dehydrogenases; Nishiyama, et al., J. Biol. Chem., 268: 4656-4660, 1993;Ferredoxin-NADP reductase, Martinez-Julvez, et al., Biophys. Chem., 115:219-224, 2005); and oxidoreductases (US2004/0248250).

Rane et al., (Arch. Biochem. Biophys., 338: 83-89, 1997) discusscofactor switching of a ketol acid reductoisomerase isolated from E.coli by targeting four residues in the enzyme for mutagenesis, (R68,K69, K75, and R76,); however the effectiveness of this method is indoubt.

Although the above cited methods suggest that it is generally possibleto switch the cofactor specificity between NADH and NADPH, the methodsare enzyme specific and the outcomes unpredictable. The development of aketol-acid reductoisomerase having a high specificity for NADH withdecreased specificity for NADPH would greatly enhance this enzyme'seffectiveness in the isobutanol biosynthetic pathway and hence increaseisobutanol production. However, no such KARI enzyme has been reported.

SUMMARY OF THE INVENTION

Applicants have solved the stated problem by identifying a number ofmutant ketol-acid reductoisomerase enzymes that either have a preferencefor specificity for NADH as opposed to NADPH or use NADH exclusively intheir reaction. The method involves mutagenesis of certain specificresidues in the KARI enzyme to produce the co-factor switching.

Accordingly the invention provides A mutant ketol-acid reductoisomeraseenzyme comprising the amino acid sequence as set forth in SEQ ID NO: 29;a nucleic acid molecule encoding a mutant ketol-acid reductoisomeraseenzyme having the amino acid sequence as set forth in SEQ ID NO:19; amutant ketol-acid reductoisomerase enzyme as set for in SEQ ID NO:19; amutant ketol-acid reductoisomerase enzyme having the amino acid sequenceselected from the group consisting of SEQ ID NO: 24, 25, 26, 27, 28, 67,68, 70, 75, 79, 80, 81 and 82; and a mutant ketol-acid reductoisomeraseenzyme as set forth in SEQ ID NO:17 comprising at least one mutation ata residue selected from the group consisting of 24, 33, 47, 50, 52, 53,61, 80, 115, 156, 165, and 170.

In another embodiment the invention provides a method for the evolutionof an NADPH binding ketol-acid reductoisomerase enzyme to an NADH usingform comprising:

-   -   a) providing a ketol-acid reductoisomerase enzyme which uses        NADPH having a specific native amino acid sequence;    -   b) identifying the cofactor switching residues in the enzyme        of (a) based on the amino acid sequence of the Pseudomonas        fluorescens ketol-acid reductoisomerase enzyme as set for the in        SEQ ID NO:17 wherein the cofactor switching residues are at        positions selected from the group consisting of: 24, 33, 47, 50,        52, 53, 61, 80, 115, 156, 165, and 170; and    -   c) creating mutations in at least one of the cofactor switching        residues of (b) to create a mutant enzyme wherein said mutant        enzyme binds NADH.

In another embodiment the invention provides a method for the productionof isobutanol comprising:

-   -   a) providing a recombinant microbial host cell comprising the        following genetic constructs:        -   i) at least one genetic construct encoding an acetolactate            synthase enzyme for the conversion of pyruvate to            acetolactate;        -   ii) at least one genetic construct encoding a ketol-acid            reductoisomerase enzyme of either of claim 1 or 6;        -   iii) at least one genetic construct encoding an acetohydroxy            acid dehydratase for the conversion of            2,3-dihydroxyisovalerate to α-ketoisovalerate, (pathway step            c);        -   iv) at least one genetic construct encoding a branched-chain            keto acid decarboxylase, of the conversion of            α-ketoisovalerate to isobutyraldehyde, (pathway step d);        -   v) at least one genetic construct encoding a branched-chain            alcohol dehydrogenase for the conversion of isobutyraldehyde            to isobutanol (pathway step e); and    -   b) growing the host cell of (a) under conditions where        iso-butanol is produced.

In another embodiment the invention provides a method for the evolutionand identification of an NADPH binding ketol-acid reductoisomeraseenzyme to an NADH using form comprising:

-   -   a) providing a ketol-acid reductoisomerase enzyme which uses        NADPH having a specific native amino acid sequence;    -   b) identifying the amino acid residues in the native amino acid        sequence whose side chains are in close proximity to the        adenosyl 2′-phosphate of NADPH as mutagenesis targets;    -   c) creating a library of mutant ketol-acid reductoisomerase        enzymes from the class I ketol-acid reductoisomerase enzyme of        step (a), having at least one mutation in at least one of the        mutagenesis target sites of step (b); and    -   d) screening the library of mutant ketol-acid reductoisomerase        enzymes of step (c) to identify NADH binding mutant of        ketol-acid reductoisomerase enzyme.

Alternatively the invention provides a method for evolution of an NADPHspecific ketol-acid reductoisomerase enzyme to an NADH using formcomprising:

-   -   a) providing a mutant enzyme having an amino acid sequence        selected from the group consisting of SEQ ID NOs: 28, 67, 68,        69, 70, and 84;    -   b) constructing a site-saturation library targeting amino acid        positions 47, 50, 52 and 53 of the mutant enzyme of (a); and    -   c) screening the site-saturation library of (b) to identify        mutants which accept NADH instead of NADPH as cofactor.

Similarly the invention provides a method for evolution of an NADPHspecific ketol-acid reductoisomerase enzyme to an NADH using formcomprising:

-   -   a) providing a DNA fragment encoding a mutant enzyme having an        amino acid sequence selected from the group consisting of SEQ ID        NOs: 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, and 98        containing mutations in cofactor specificity domain;    -   b) producing a DNA fragment cofactor specificity domain of (a);    -   c) providing a DNA fragment encoding a mutant enzyme having        mutations in cofactor binding affinity domain selected from the        group consisting of SEQ ID NOs: 28, 67, 68, 69, 70, 84 and 86;    -   d) incorporating mutations of step (b) into mutants of step (c);        and    -   e) screening mutants of step (d) for mutant enzymes having a        ratio of NADH/NADPH utilization is greater than one.

BRIEF DESCRIPTION OF THE FIGURES AND SEQUENCE DESCRIPTIONS

The invention can be more fully understood from the following detaileddescription, the Figures, and the accompanying sequence descriptions,which form part of this application.

FIGS. 1A and 1B—Show four different isobutanol biosynthetic pathways.The steps labeled “a”, “b”, “c”, “d”, “f”, “g”, “h”, “i”, “j” and “k”represent the substrate to product conversions described below.

FIGS. 2A and 2B—Multiple sequence alignment (MSA) of KARI enzymes fromdifferent recourses; FIG. 2A—MSA among three NADPH-requiring KARIenzymes; FIG. 2B—MSA among PF5-KARI and other KARI enzymes, withpromiscuous nucleotide specificity, where, MMC5—is from Methanococcusmaripaludis C5; MMS2—is from Methanococcus maripaiudis S2; MNSB—is fromMethanococcus vanniellii SB; ilv5—is from Saccharomyces corevisiae ilv5;KARI-D1—is from Sulfolobus solfataricus P2 ilvC; KARI-D2—is fromPyrobaculum aerophilum P2ilvC; and KARI S1—is from Raistoniasolanacearum GMI1000 ivlC.

FIG. 3—Interaction of phosphate binding loop with NADPH based onhomology modeling.

FIG. 4—KARI activities of top performers from library C using cofactorNADH versus NADPH. Activity and standard deviation were derived fromtriple experiments. The mutation information is as follows:C3A7=R47Y/S50A/T52D/V53W; C3A10=R47Y/S50A/T52G/V53W;C3B11=R47F/S50A/T52DA/53W; C3C8=R47G/S50M/T52D/V53W; andC4D12=R47C/S50MT52D/V53W

FIGS. 5A and 5B—FIG. 5A-Comparison of KARI activities of top performersfrom libraries E, F and G using cofactors NADH and NADPH. FIG. 5B—KARIactivities of positive control versus wild type Pf5-ilvC using cofactorsNADH. Activity and standard deviation were derived from at least threeparallel experiments. “Wt” represents the wild type of Pf5-ilvC and“Neg” means negative control. Experiments for NADH and NADPH reactionsin FIG. 5A were 30 min; in FIG. 5B were 10 min.

FIG. 6—Activities of top performers from library H using cofactors NADHversus NADPH. Activity and standard deviation were derived from tripleexperiments. Mutation information is as follows: 24F9=R47P/S50G/T52D;68F10=R47P/T52S; 83G10=R47P/S50D/T52S; 39G4=R47P/S50C/T52D;91A9=R47P/S50CT52D; and C3B11=R47F/S50A/T52D/V53W and Wt is wild type.

FIG. 7—Thermostability of wild type PF5-ilvC. The remaining activity ofthe enzyme after heating at certain temperatures for 10 min was theaverage number of triple experiments and normalized to the activitymeasured at room temperature.

FIG. 8—Multiple DNA sequence alignment among 5 naturally existing KARImolecules. The positions both bolded and boxed were identified by errorprone PCR and the positions only boxed were targeted for mutagenesis.

FIGS. 9A through 9k—Alignment of the twenty-four functionally verifiedKARI sequences. The GxGXX(G/A) motif involved in the binding of NAD(P)His indicated below the alignment.

FIGS. 10A and 10B—An example of the alignment of Pseudomonas fluorescensPf-5 KARI to the profile HIM of KARI. The eleven positions that areresponsible for co-factor switching are boxed.

FIG. 11—(A) is a linear depiction of the KARI amino acid sequence withspecific amino acids numbered. The cofactor specificity domain residuesare shown in shaded rectangles. The cofactor binding domain is shown indotted ovals. (Table A) shows changed amino acids, using single lettercode, at numbered positions in four KARI mutants.

FIG. 12 (A) is a linear depiction of the KARI amino acid sequence withspecific amino acids numbered. The cofactor specificity domain residuesare shown in shaded rectangles. (B) Depicts the first PCR stepamplifying the mutated cofactor specificity domain residues. (C) is alinear depiction of the KARI amino acid sequence with specific aminoacids of the cofactor binding domain shown in dotted ovals. (D) Depictsincorporation of the domain swapping library into the mutants containingK_(M) improving mutations. Table (E) summaries the K_(M) values for NADHfor mutations resulting from combining mutations in the cofactor bindingaffinity domain with mutations in the cofactor specificity determiningdomain.

Table 9—is a table of the Profile HMI of the KARI enzymes described inExample 3. The eleven positions in the profile HMM representing thecolumns in the alignment which correspond to the eleven cofactorswitching positions in Pseudomonas fluorescens Pf-5 KARI are identifiedas positions 24, 33, 47, 50, 52, 53, 61, 80, 115, 156, and 170. Thelines corresponding to these positions in the model file are highlightedin yellow. Table 9 is submitted herewith electronically and isincorporated herein by reference.

The following sequences conform with 37 C.F.R. 1.821-1.825(“Requirements for Patent Applications Containing Nucleotide Sequencesand/or Amino Acid Sequence Disclosures—the Sequence Rules”) and areconsistent with the World Intellectual Property Organization (WIPO)Standard ST.25 (1998) and the sequence listing requirements of the EPOand PCT (Rules 5.2 and 49.5(a-bis), and Section 208 and Annex C of theAdministrative Instructions). The symbols and format used for nucleotideand amino acid sequence data comply with the rules set forth in 37C.F.R.§1.822.

TABLE 1 Oligonucleotide Primers Used In This Invention SEQUENCE ID No.SEQUENCE Description 1 TGATGAACATCTTCGCGTATTCGCCGTCCT Reverse Primer forpBAD vector 2 GCGTAGACGTGACTGTTGGCCTGNNTAAAGGCNN Forward primerGGCTNNCTGGGCCAAGGCT GAAGCCCACGGCTTG library C 3GCGTAGACGTGACTGTTGGCCTGNNTAAAGGCTCG Forward primer forGCTACCGTTGCCAAGGCTGAAGCCCACGGCTTG library E 4GCGTAGACGTGACTGTTGGCCTGCGTAAAGGCNNT Forward primer forGCTACCGTTGCCAAGGCTGAAGCCCACGGCTTG library F 5GCGTAGACGTGACTGTTGGCCTGCGTAAAGGCTCG Forward primer forGCTNNTGTTGCCAAGGCTGAAGCCCACGGCTTG library G 6GCGTAGACGTGACTGTTGGCCTGNNTAAAGGCNNT Forward primer forGCTNNTGTTGCCAAGGCTGAAGCCCACGGCTTG library H 7 AAGATTAGCGGATCCTACCTSequencing primer (forward) 8 AACAGCCAAGCTTTTAGTTC Sequencing primer(reverse) 20 CTCTCTACTGTTTCTCCATACCCG pBAD_266-021308f 21CAAGCCGTGGGCTTCAGCCTTGGCKNN PF5_53Mt022908r 22 CGGTTTCAGTCTCGTCCTTGAAGpBAD_866-021308 49 GCTCAAGCANNKAACCTGAAGG pBAD-405- C33_090808f 50CCTTCAGGTTKNNTGCTTGAGC pBAD-427- C33_090808r 51 GTAGACGTGNNKGTTGGCCTGpBAD-435- T43_090808f 52 CAGGCCAACKNNCACGTCTAC pBAD-456- T43_090808r 53CTGAAGCCNNKGGCNNKAAAGTGAC pBAD-484- H59L61_090808f 54GTCACTTTKNNGCCKNNGGCTTCAG pBAD-509- H59L61_090808r 55GCAGCCGTTNNKGGTGCCGACT pBAD-519- A71_090808f 56 AGTCGGCACCKNNAACGGCTGCpBAD-541- A71_090808r 57 CATGATCCTGNNKCCGGACGAG pBAD-545- T80_090808f 58CTCGTCCGGKNNCAGGATCATG pBAD-567- T80_090808r 59 CAAGAAGGGCNNKACTCTGGCCTpBAD-608- A101_090808f 60 AGGCCAGAGTKNNGCCCTTCTTG pBAD-631- A101_090808r61 GTTGTGCCTNNKGCCGACCTCG pBAD-663- R119_090808f 62CGAGGTCGGCKNNAGGCACAAC pBAD-685- R119_090808r 71GTAGACGTGACTGTTGGCCTGNNKAAAGGCNNKGC PF5_4Mt111008.fTNNKNNKGCCAAGGCTGAAGCCCACGG 72 CCGTGGGCTTCAGCCTTGGCKNNKNNAGCKNNGCPF5_4Mt111008.r CTTTKNNCAGGCCAACAGTCACGTCTAC 73 AAGATTAGCGGATCCTACCTpBAD_230.f 74 GAGTGGCGCCCTTCTTGATGTTCG pBAD_601_021308r

Additional sequences used in the application are listed below. Theabbreviated gene names in bracket are used in this disclosure.

SEQ ID NO: 9-Methanococcus maripaludis C5-ilvC (MMC5)-GenBank AccessionNumber NC_(—)009135.1 Region: 901034.902026SEQ ID NO: 10 is the Methanococcus maripaludis S2-ilvC (MMS2)-GenBankAccession Number NC_(—)005791.1 Region: 645729.646721SEQ ID NO: 11 is the Methanococcus vannielii SB-ilv5 (MVSB)-GenBankAccession Number NZ_AAWX01000002.1 Region: 302214.303206SEQ ID NO: 12 is the Saccharomyces cerevisiae 11v5 (ilv5)-GenBankAccession Number NC_(—)001144.4 Region: 838065.839252SEQ ID NO: 13 is the Sulfolobus solfalaricus P2 ilvC (KARI-D1)-GenBankAccession Number NC_(—)002754.1 Region: 506253.507260SEQ ID NO: 14 is the Pyrobaculum aerophilum str. IM2 ilvC (KARI-D2)GenBank Accession Number NC_(—)003364.1 Region: 1976281.1977267SEQ ID NO: 15 is the Raistonia solanacearum GM11000 ilvC (KARI-S1)GenBank Accession Number NC_(—)003295.1 Region: 2248264.2249280SEQ ID NO: 16 is the Pseudomonas aeruginosa PAO1 ilvC GenBank AccessionNumber NC_(—)002516 Region: 5272455.5273471SEQ ID NO: 17 is the Pseudomonas fluorescens PF5 ilvC-GenBank AccessionNumber NC_(—)004129 Region: 6017379.6018395SEQ ID NO: 18 is the Spinacia oleracea ilvC (Spinach-KARI)-GenBankAccession Number NC_(—)002516 Region: 1.2050.SEQ ID NO: 19 is the amino acid sequence of the mutant(Y24F/R47Y/S50A/T52D1V53A/L61F/G170A) of the ilvC native protein ofPseudomonas fluorescens.SEQ ID NO: 23 is the DNA SEQ of the mutant(Y24FIR47Y/S50A/T52D/V53A/L61F/G170A) of the ilvC native protein ofPseudomonas fluorescens.SEQ ID NO: 24 is the amino acid SEQ of the mutant ZB1(Y24F/R47Y/S50A/T52D/V53A/L61F/A156V)SEQ ID NO: 25 is the amino acid SEQ of the mutant ZF3(Y24F/C33L/R47Y/S50A/T52D/V53A/L61F)SEQ ID NO: 26 is the amino acid SEQ of the mutant ZF2(Y24F/C33L/R47Y/S50A/T52D/V53A/L61F/A156V)SEQ ID NO: 27 is the amino acid SEQ of the mutant ZB3(Y24F/C33L/R47Y/S50A/T52D/V53A/L61F/G170A)SEQ ID NO: 28 is the amino acid SEQ of the mutant Z4B8(C33L/R47Y/S50A/T52D/V53A/L61F/T80I/A156V/G170A)SEQ ID NO: 29 is a consensus amino acid sequence comprising allexperimentally verified KARI point mutations as based on SEQ ID NO:17.SEQ ID NO: 30 is the amino acid sequence for KARI from Natronomonaspharaonis DSM 2160SEQ ID NO: 31 is the amino acid sequence for KARI from Bacillus subtilissubsp. subtilis str. 168SEQ ID NO: 32 is the amino acid sequence for KARI from Corynebacteriumglutamicurn ATCC13032SEQ ID NO: 33 is the amino acid sequence for KARI from PhaeospirilummolischianumSEQ ID NO: 34 is the amino acid sequence for KARI from Zymomonas mobilissubsp. mobilis ZM4SEQ ID NO: 35 is the amino acid sequence for KARI Alkalilimnicolaehrlichei MLHE-1SEQ ID NO: 36 is the amino acid sequence for KARI from Campylobacterlari RM2100SEQ ID NO: 37 is the amino acid sequence for KARI from Marinobacteraquaeolei VT8SEQ ID NO: 38 is the amino acid sequence for KARI Psychrobacter arcticus273-4SEQ ID NO: 39 is the amino acid sequence for KARI from Hahellachejuensis KCTC2396SEQ ID NO: 40 is the amino acid sequence for KARI from Thiobacillusdenitrificans ATCC25259SEQ ID NO: 41 is the amino acid sequence for KARI from Azotobactervinelandii AvOPSEQ ID NO: 42 is the amino acid sequence for KARI from Pseudomonassyringae pv, syringae B728aSEQ ID NO: 43 is the amino acid sequence for KARI from Pseudomonassyringae pv. tomato str. DC3000SEQ ID NO: 44 is the amino acid sequence for KARI from Pseudomonasputida KT2440SEQ ID NO: 45 is the amino acid sequence for KARI from Pseudomonasentomophila L48SEQ ID NO: 46 is the amino acid sequence for KARI from Pseudomonasmendocina ympSEQ ID NO: 47 is the amino acid sequence for KARI from Bacillus cereusATCC 10987 NP_(—)977840.1SEQ ID NO: 48 is the amino acid sequence for KARI from Bacillus cereusATCC10987 NP_(—)978252.1SEQ ID NO: 63 is the amino acid sequence for KARI from Escherichiacoli-GenBank Accession Number P05793SEQ ID NO: 64 is the amino acid sequence for KARI from Marine GammaProteobacterium HTCC2207 GenBank Accession Number ZP_(—)01224863.1SEQ ID NO: 65 is the amino acid sequence for KARI from Desulfuromonasacetoxidans-GenBank Accession Number ZP_(—)01313517iSEQ ID NO: 66 is the amino acid sequence for KARI from Pisum sativum(Pea)-GenBank Accession Number O82043SEQ ID NO: 67 is the amino acid sequence for mutant 3361G8(C33L/R47Y/S50A/T52D/V53A/L61F/T80I)SEQ ID NO: 68 is the amino acid sequence for mutant 2H10(Y24FIC33L/R47Y/S50A/T52DA/531I/L61F/T80I/A156V)SEQ ID NO: 69 is the amino acid sequence for mutant 1D2(Y24F/R47Y/S50A/T52D/V53A/L61F/T80I/A156V.SEQ ID NO: 70 is the amino acid sequence for mutant 3F12(Y24F/C33L/R47Y/S50A/T52DA/53A/L61F/T80I/A156V).SEQ ID NO: 75 is the amino acid sequence for mutant JB1C6(Y24F/C33L/R47H/S50D/T52YA/53Y/L61F/T80I/A156V)SEQ ID NO: 76 is the amino acid sequence for mutant 16445E4(C33L/R47P/S50V/T52D/V53G/L61F/T80I/A156V)SEQ ID NO: 77 is the amino acid sequence for mutant 16468D7(Y24F/C33L/R47T/S50I/T52D/V53R/L61F/T80I/A156V)SEQ ID NO: 78 is the amino acid sequence for mutant 16469F3(C33L/R47E/S50A/T52D/V53A/L61F/T80I)SEQ ID NO: 79 is the amino acid sequence for mutant JEA1(Y24F/C33L/R47R/S50F/T52D/L61F/T80I/A156V)SEQ ID NO: 80 is the amino acid sequence for mutant JEG2(Y24F/C33L/R47F/S50A/T52DA/53A/L61F/T80I/A156V)SEQ ID NO: 81 is the amino acid sequence for mutant JEG4(Y24FIC33L/R47N/S50N/T52D/V53A/L61F/T80I/A56V)SEQ ID NO: 82 is the amino acid sequence for mutant JEA7 (Y24FIC33L/R47P/S50N/T52D/V53A/L61F/T80I/A156V)SEQ ID NO: 83 is the amino acid sequence for mutant JED1(C33L/R47N/S50N/T52D/V53A/L61F/T80I/A156V)SEQ ID NO: 84 is the amino acid sequence for mutant 3361E1SEQ ID NO: 85 is the amino acid sequence for mutant C2F6SEQ ID NO: 86 is the amino acid sequence for mutant C3B11SEQ ID NO: 87 is the amino acid sequence for mutant C4D12SEQ ID NO: 88 is the amino acid sequence for mutant SE1SEQ ID NO: 89 is the amino acid sequence for mutant SE2SEQ ID NO: 90 is the amino acid sequence for mutant SB3SEQ ID NO: 91 is the amino acid sequence for mutant SD3SEQ ID NO: 92 is the amino acid sequence for mutant 9650E5SEQ ID NO: 93 is the amino acid sequence for mutant 9667A11SEQ ID NO: 94 is the amino acid sequence for mutant 9862B9SEQ ID NO: 95 is the amino acid sequence for mutant 9875B9SEQ ID NO: 96 is the amino acid sequence for mutant 11461D8SEQ ID NO: 97 is the amino acid sequence for mutant 11463SEQ ID NO: 98 is the amino acid sequence for mutant 11518B4

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the generation of mutated KARI enzymesto use NADH as opposed to NADPH. Such co-factor switched enzymesfunction more effectively in microbial systems designed to produceIsobutanol. Isobutanol is an important industrial commodity chemicalwith a variety of applications, where its potential as a fuel or fueladditive is particularly significant. Although only a four-carbonalcohol, butanol has the energy content similar to that of gasoline andcan be blended with any fossil fuel. Isobutanol is favored as a fuel orfuel additive as it yields only CO₂ and little or no SO_(x) or NO_(x)when burned in the standard internal combustion engine. Additionallybutanol is less corrosive than ethanol, the most preferred fuel additiveto date.

The following definitions and abbreviations are to be use for theinterpretation of the claims and the specification.

The term “invention” or “present invention” as used herein is meant toapply generally to all embodiments of the invention as described in theclaims as presented or as later amended and supplemented, or in thespecification.

The term “isobutanol biosynthetic pathway” refers to the enzymaticpathway to produce isobutanol. Preferred isobutanol biosyntheticpathways are illustrated in FIG. 1 and described herein.

The term “NADPH consumption assay” refers to an enzyme assay for thedetermination of the specific activity of the KARI enzyme, involvingmeasuring the disappearance of the KARI cofactor, NADPH, from the enzymereaction.

“KARI” is the abbreviation for the enzyme ketol-acid reductoisomerase.

The term “close proximity” when referring to the position of variousamino acid residues of a KARI enzyme with respect to the adenosyl2′-phosphate of NADPH means amino acids in the three-dimensional modelfor the structure of the enzyme that are within about 4.5 Å of thephosphorus atom of the adenosyl 2′-phosphate of NADPH bound to theenzyme.

The term “ketol-acid reductoisomerase” (abbreviated “KARI”), and“acetohydroxy acid isomeroreductase” will be used interchangeably andrefer to the enzyme having the EC number, EC 1.1.1.86 (EnzymeNomenclature 1992, Academic Press, San Diego). Ketol-acidreductoisomerase catalyzes the reaction of (S)-acetolactate to2,3-dihydroxyisovalerate, as more fully described below. These enzymesare available from a number of sources, including, but not limited to E.coli GenBank Accession Number NC-000913 REGION: 3955993.3957468, Vibriocholerae GenBank Accession Number NC-002505 REGION: 157441.158925,Pseudomonas aeruginosa, GenBank Accession Number NC-002516. (SEQ ID NO:16) REGION: 5272455.5273471, and Pseudomonas fluorescens GenBankAccession Number NC-004129 (SEQ ID NO: 17) REGION: 6017379.6018395. Asused herein the term “Class I ketol-acid reductoisomerase enzyme” meansthe short form that typically has between 330 and 340 amino acidresidues, and is distinct from the long form, called class II, thattypically has approximately 490 residues.

The term “acetolactate synthase” refers to an enzyme that catalyzes theconversion of pyruvate to acetolactate and CO₂.

Acetolactate has two stereoisomers ((R) and (3)); the enzyme prefers the(S)-isomer, which is made by biological systems. Preferred acetolactatesynthases are known by the EC number 2.2.1.6 9 (Enzyme Nomenclature1992, Academic Press, San Diego). These enzymes are available from anumber of sources, including, but not limited to, Bacillus subtilis(GenBank Nos: CAB15618, Z99122, NCBI (National Center for BiotechnologyInformation) amino acid sequence, NCBI nucleotide sequence,respectively), klebsiella pneumoniae (Gen Bank Nos: AAA25079, M73842 andLactococcus lactic (GenBank Nos: AAA25161, L16975).

The term “acetohydroxy acid dehydratase” refers to an enzyme thatcatalyzes the conversion of 2,3-dihydroxyisovalerate toα-ketoisovalerate. Preferred acetohydroxy acid dehydratases are known bythe EC number 4.2.1.9. These enzymes are available from a vast array ofmicroorganisms, including, but not limited to, E. coli (GenBank Nos:YP_(—)026248, NC_(—)000913, S. cerevisiae (GenBank Nos: NP_(—)012550,NC_(—)001142), M. maripaludis (GenBank Nos: CAF29874, BX957219), and B.subtilis (GenBank Nos: CAB14105, Z99115).

The term “branched-chain α-keto acid decarboxylase” refers to an enzymethat catalyzes the conversion of α-ketoisovalerate to isobutyraldehydeand CO₂. Preferred branched-chain α-keto acid decarboxylases are knownby the EC number 4.1.1.72 and are available from a number of sources,including, but not limited to, Lactococcus lactic (GenBank Nos:AAS49166, AY548760; CAG34226, AJ746364, Salmonella typhimurium (GenBankNos: NP-461346, NC-003197), and Clostridium acetobutylicum (GenBank Nos:NP-149189, NC-001988).

The term “branched-chain alcohol dehydrogenase” refers to an enzyme thatcatalyzes the conversion of isobutyraldehyde to isobutanol. Preferredbranched-chain alcohol dehydrogenases are known by the EC number1.1.1.265, but may also be classified under other alcohol dehydrogenases(specifically, EC 1.1.1.1 or 1.1.1.2). These enzymes utilize NADH(reduced nicotinamide adenine dinucleotide) and/or NADPH as electrondonor and are available from a number of sources, including, but notlimited to, S. cerevisiae (GenBank Nos: NP-010656, NC-001136; NP-014051,NC-001145), E. coli (GenBank Nos: NP-417484, and C. acetobutylicum(GenBank Nos: NP-349892, NC_(—)003030).

The term “branched-chain keto acid dehydrogenase” refers to an enzymethat catalyzes the conversion of α-ketoisovalerate to isobutyryl-CoA(isobutyryl-cofactor A), using NAD⁺ (nicotinamide adenine dinucleotide)as electron acceptor. Preferred branched-chain keto acid dehydrogenasesare known by the EC number 1.2.4.4. These branched-chain keto aciddehydrogenases comprise four subunits, and sequences from all subunitsare available from a vast array of microorganisms, including, but notlimited to, B. subtilis (GenBank Nos: CAB14336, Z99116; CAB14335,Z99116; CAB14334, Z99116; and CAB14337, Z99116) and Pseudomonas putida(GenBank Nos: AAA65614, M57613; AAA65615, M57613; AAA65617, M57613; andAAA65618, M57613).

The terms “k_(cat)” and “K_(M)” are known to those skilled in the artand are described in Enzyme Structure and Mechanism, 2nd ed. (Ferst;Freeman Press, NY, 1985; pp 98-120). The term “k_(cat)”, often calledthe “turnover number”, is defined as the maximum number of substratemolecules converted to products per active site per unit time, or thenumber of times the enzyme turns over per unit time.K_(cat)=V_(max)/[E], where [E] is the enzyme concentration (Ferst,supra). The terms “total turnover” and “total turnover number” are usedherein to refer to the amount of product formed by the reaction of aKARI enzyme with substrate.

The term “catalytic efficiency” is defined as the K_(cat)/K_(m) of anenzyme. Catalytic efficiency is used to quantify the specificity of anenzyme for a substrate.

The term “isolated nucleic acid molecule”, “isolated nucleic acidfragment” and “genetic construct” will be used interchangeably and willmean a polymer of RNA or DNA that is single- or double-stranded,optionally containing synthetic, non-natural or altered nucleotidebases. An isolated nucleic acid fragment in the form of a polymer of DNAmay be comprised of one or more segments of cDNA, genomic DNA orsynthetic DNA.

The term “amino acid” refers to the basic chemical structural unit of aprotein or polypeptide. The following abbreviations are used herein toidentify specific amino acids:

Three-Letter One-Letter Amino Acid Abbreviation Abbreviation Alanine AlaA Arginine Arg R Asparagine Asn N Aspartic acid Asp D Cysteine Cys CGlutamine Gln Q Glutamic acid Glu E Glycine Gly G Histidine His HLeucine Leu L Lysine Lys K Methionine Met M Phenylalanine Phe F ProlinePro P Serine Ser S Threonine Thr T Tryptophan Trp W Tyrosine Tyr YValine Val V

The term “gene” refers to a nucleic acid fragment that is capable ofbeing expressed as a specific protein, optionally including regulatorysequences preceding (5′ non-coding sequences) and following (3′non-coding sequences) the coding sequence. “Native gene” refers to agene as found in nature with its own regulatory sequences. “Chimericgene” refers to any gene that is not a native gene, comprisingregulatory and coding sequences that are not found together in nature.Accordingly, a chimeric gene may comprise regulatory sequences andcoding sequences that are derived from different sources, or regulatorysequences and coding sequences derived from the same source, butarranged in a manner different than that found in nature. “Endogenousgene” refers to a native gene in its natural location in the genome of amicroorganism. A “foreign” gene refers to a gene not normally found inthe host microorganism, but that is introduced into the hostmicroorganism by gene transfer. Foreign genes can comprise native genesinserted into a non-native microorganism, or chimeric genes. A“transgene” is a gene that has been introduced into the genome by atransformation procedure.

As used herein the term “coding sequence” refers to a DNA sequence thatencodes for a specific amino acid sequence. “Suitable regulatorysequences” refer to nucleotide sequences located upstream (5′ non-codingsequences), within, or downstream (3′ non-coding sequences) of a codingsequence, and which influence the transcription, RNA processing orstability, or translation of the associated coding sequence. Regulatorysequences may include promoters, translation leader sequences, introns,polyadenylation recognition sequences, RNA processing site, effectorbinding site and stem-loop structure.

The term “promoter” refers to a DNA sequence capable of controlling theexpression of a coding sequence or functional RNA. In general, a codingsequence is located 3′ to a promoter sequence. Promoters may be derivedin their entirety from a native gene, or be composed of differentelements derived from different promoters found in nature, or evencomprise synthetic DNA segments. It is understood by those skilled inthe art that different promoters may direct the expression of a gene indifferent tissues or cell types, or at different stages of development,or in response to different environmental or physiological conditions,Promoters which cause a gene to be expressed in most cell types at mosttimes are commonly referred to as “constitutive promoters”. It isfurther recognized that since in most cases the exact boundaries ofregulatory sequences have not been completely defined, DNA fragments ofdifferent lengths may have identical promoter activity.

The term “operably linked” refers to the association of nucleic acidsequences on a single nucleic acid fragment so that the function of oneis affected by the other. For example, a promoter is operably linkedwith a coding sequence when it is capable of effecting the expression ofthat coding sequence (i.e., that the coding sequence is under thetranscriptional control of the promoter). Coding sequences can beoperably linked to regulatory sequences in sense or antisenseorientation.

The term “expression”, as used herein, refers to the transcription andstable accumulation of sense (mRNA) or antisense RNA derived from thenucleic acid fragment of the invention. Expression may also refer totranslation of mRNA into a polypeptide.

As used herein the term “transformation” refers to the transfer of anucleic acid fragment into the genome of a host microorganism, resultingin genetically stable inheritance. Host microorganisms containing thetransformed nucleic acid fragments are referred to as “transgenic” or“recombinant” or “transformed” microorganisms.

The terms “plasmid”, “vector” and “cassette” refer to an extrachromosomal element often carrying genes which are not part of thecentral metabolism of the cell, and usually in the form of circulardouble-stranded DNA fragments. Such elements may be autonomouslyreplicating sequences, genome integrating sequences, phage or nucleotidesequences, linear or circular, of a single- or double-stranded DNA orRNA, derived from any source, in which a number of nucleotide sequenceshave been joined or recombined into a unique construction which iscapable of introducing a promoter fragment and DNA sequence for aselected gene product along with appropriate 3′ untranslated sequenceinto a cell. “Transformation cassette” refers to a specific vectorcontaining a foreign gene and having elements in addition to the foreigngene that facilitates transformation of a particular host cell.“Expression cassette” refers to a specific vector containing a foreigngene and having elements in addition to the foreign gene that allow forenhanced expression of that gene in a foreign host,

The term “site-saturation library” refers to a library which containsrandom substitutions at a specific amino acid position with all 20possible amino acids at once.

The term “error-prone PCR” refers to adding random copying errors byimposing imperfect or ‘sloppy’ PCR reaction conditions which generaterandomized libraries of mutations in a specific nucleotide sequence.

As used herein the term “codon degeneracy” refers to the nature in thegenetic code permitting variation of the nucleotide sequence withoutaffecting the amino acid sequence of an encoded polypeptide. The skilledartisan is well aware of the “codon-bias” exhibited by a specific hostcell in usage of nucleotide codons to specify a given amino acid.Therefore, when synthesizing a gene for improved expression in a hostcell, it is desirable to design the gene such that its frequency ofcodon usage approaches the frequency of preferred codon usage of thehost cell.

The term “codon-optimized” as it refers to genes or coding regions ofnucleic acid molecules for transformation of various hosts, refers tothe alteration of codons in the gene or coding regions of the nucleicacid molecules to reflect the typical codon usage of the hostmicroorganism without altering the polypeptide encoded by the DNA.

Molecular Techniques

Standard recombinant DNA and molecular cloning techniques used here arewell known in the art and are described by Sambrook et al. (Sambrook,Fritsch and Maniatis, Molecular Cloning: A Laboratory Manual, SecondEdition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.,1989) (hereinafter “Maniatis”); and by Silhavy et al. (Experiments withGene Fusions, Cold Spring Harbor Laboratory Press Cold Spring Harbor,N.Y., 1984); and by Ausubel, F. M. et al., (Current Protocols inMolecular Biology, published by Greene Publishing Assoc. andWiley-Interscience, 1987).

The present invention addresses a need that arises in the microbialproduction of isobutanol where the ketol-acid reductoisomerase enzymeperforms a vital role. Wild type ketol-acid reductoisomerase enzymestypically use NADPH as their cofactor. However, in the formation ofisobutanol an excess of NADH is produced by ancillary metabolicpathways. The invention provides mutant Class I KARI enzymes that havebeen evolved to utilize NADH as a cofactor, overcoming the cofactorproblem and increasing the efficiency of the isobutanol biosyntheticpathway.

Production of isobutanol utilizes the glycolysis pathway present in thehost microorganism. During the production of two molecules of pyruvatefrom glucose during glycolysis, there is net production of two moleculesof NADH from NAD⁺ by the glyceraldehyde-3-phosphate dehydrogenasereaction. During the further production of one molecule of isobutanolfrom two molecules of pyruvate, there is net consumption of one moleculeof NADPH, by the KARI reaction, and one molecule of NADH by theisobutanol dehydrogenase reaction. The overall reaction of glucose toisobutanol thus leads to net production of one molecule of NADH and netconsumption of one molecule of NADPH. The interconversion of NADH withNADPH is generally slow and inefficient; thus, the NADPH consumed isgenerated by metabolism (for example, by the pentose phosphate pathway)consuming substrate in the process. Meanwhile, the cell strives tomaintain homeostasis in the NAD⁺/NADH ratio, leading to the excess NADHproduced in isobutanol production being consumed in wasteful reductionof other metabolic intermediates; e.g., by the production of lactatefrom pyruvate. Thus, the imbalance between NADH produced and NADPHconsumed by the isobutanol pathway leads to a reduction in the molaryield of isobutanol produced from glucose in two ways: 1) unnecessaryoperation of metabolism to produce NADPH, and 2) wasteful reaction ofmetabolic intermediates to maintain NAD″/NADH homeostasis. The solutionto this problem is to invent a KARI that is specific for NADH as itscofactor, so that both molecules of NADH produced in glycolysis areconsumed in the synthesis of isobutanol from pyruvate.

Keto Acid Reductoisomerase (KARI) Enzymes

Acetohydroxy acid isomeroreductase or ketol-acid reductoisomerase (KARI;EC 1.1.1.86) catalyzes two steps in the biosynthesis of branched-chainamino adds and is a key enzyme in their biosynthesis. KARI is found in avariety of microorganisms and amino acid sequence comparisons acrossspecies have revealed that there are 2 types of this enzyme: a shortform (class I) found in fungi and most bacteria, and a long form (classII) typical of plants.

Class I KARIs typically have between 330-340 amino acid residues. Thelong form KARI enzymes have about 490 amino acid residues. However, somebacteria such as Escherichia coli possess a long form, where the aminoacid sequence differs appreciably from that found in plants. KARI isencoded by the ilvC gene and is an essential enzyme for growth of E.coli and other bacteria in a minimal medium. Typically KARI uses NADPHas cofactor and requires a divalent cation such as Mg⁺⁺ for itsactivity. In addition to utilizing acetolactate in the valine pathway,KARI also converts acetohydroxybutanoate to dihydroxymethylpentanoate inthe isoleucine production pathway.

Class II KARIs generally consist of a 225-residue N-terminal domain anda 287-residue C-terminal domain. The N-terminal domain, which containsthe NADPH-binding site, has an α/β structure and resembles domains foundin other pyridine nucleotide-dependent oxidoreductases. The C-terminaldomain consists almost entirely of helices and is of a previouslyunknown topology.

The crystal structure of the E. coli KARI enzyme at 2.6 Å resolution hasbeen solved (Tyagi, et al., Protein Sci., 14: 3089-3100, 2005). Thisenzyme consists of two domains, one with mixed cLIp structure which issimilar to that found in other pyridine nucleotide-dependentdehydrogenases. The second domain is mainly α-helical and shows strongevidence of internal duplication. Comparison of the active sites of KARIof E. coli. Pseudomonas aeruginosa, and spinach showed that mostresidues in the active site of the enzyme occupy conserved positions.While the E. coli KARI was crystallized as a tetramer, which is probablythe likely biologically active unit, the P. aeruginosa KARI (Ahn, etal., J. Mol. Biol., 328: 505-515, 2003) formed a dodecamer, and theenzyme from spinach formed a dimer. Known KARIs are slow enzymes with areported turnover number (k_(cat)) of 2 s⁻¹ (Aulabaugh et al.;Biochemistry, 29: 2824-2830, 1990) or 0.12 s⁻¹ (Rene et al., Arch.Biochem. Biophys. 338: 83-89, 1997) for acetolactate. Studies have shownthat genetic control of isoleucine-valine biosynthesis in E. coli isdifferent than that in Ps. aeruginosa (Marinus, et al., Genetics, 63:547-56, 1969).

Identification of Amino Acid Target Sites for Cofactor Switching

It was reported that phosphate p2″ oxygen atoms of NADPH form hydrogenbonds with side chains of Arg162, Ser165 and Ser167 of spinach KARI(Bion V., et al. The EMBO Journal, 16: 3405-3415, 1997). Multiplesequence alignments were performed, using vector NTI (Invitrogen Corp.Carlsbad, Calif.), with KARI enzymes from spinach, Pseudomonasaeruginosa (PAO-KARI) and Pseudomonas fluorescens (PF5-KARI). The NADPHbinding sites are shown in FIG. 2A. The amino acids, argenine, threonineand serine appear to play similar roles in forming hydrogen bonds withphosphate p2′ oxygen atoms of NADPH in KARI enzymes. Studies by Ahn etal., (J. Mol. Biol., 328: 505-515, 2003) had identified three NADPHphosphate binding sites (Arg47, Ser50 and Thr52) for Pseudomonasaeruginosa (PAO-KARI) following comparing its structure with that of thespinach KARI. Hypothesizing that these three NADPH phosphate bindingsites of the three KARI enzymes used in the disclosure were conserved,Arg47, Ser50 and Thr52 of PF5-KARI were targeted as the phosphatebinding sites for this enzyme. This hypothesis was further confirmedthrough homology modeling.

Multiple sequence alignment among PF5-ilvC and several other KARIenzymes with promiscuous nucleotide specificity was also performed. Asshown in FIG. 2B, the amino acids of glycine (G50) and tryptophan (W53),in other KARI enzymes in FIG. 2B, always appear together as a pair inthe sequences of those enzymes. It was therefore assumed that thetryptophan 53 bulky residue was important in determining nucleotidespecificity and by reducing the size of nucleotide binding pocket onecould favor binding of the smaller nucleotide, NADH. Position 53 ofPF5-ilvC was therefore chosen as a target for mutagenesis.

Several site-saturation gene libraries were prepared containing genesencoding KARI enzymes by commercially available kits for the generationof mutants. Clones from each library were screened for improved KARIactivity using the NADH consumption assay described herein. Screeningresulted in the identification of a number of genes having mutationsthat can be correlated to KARI activity. The location of the mutationswere identified using the amino acid sequence of the Pseudomonasfluorescens PF5/NC protein (SEQ ID NO:17). Mutants with improved KARIactivity had mutations at one or more positions at amino acids: 24, 33,47, 50, 52, 53, 61, 80, 115, 156, 165, and 170. More specificallydesirable mutations included the following substitutions:

-   -   a) the residue at position 47 has an amino acid substitution        selected from the group consisting of A, C, D, F, G, I, L, N, P,        and Y;    -   b) the residue at position 50 has an amino acid substitution        selected from the group consisting of A, C, D, E, F, G, M, N, V,        W;    -   c) the residue at position 52 has an amino acid substitution        selected from the group consisting of A, C, D, G, H, N, W;    -   d) the residue at position 53 has an amino acid substitution        selected from the group consisting of A, H, I, W;

In another embodiment, additional mutagenesis, using error prone PCR,performed on the mutants listed above identified suitable mutationpositions as: 156, 165, 61, 170, 115 and 24. More specifically thedesirable mutants with lower K_(M) for NADH contained the followingsubstitutions:

-   -   e) the residue at position 156 has an amino acid substitution of        V;    -   f) the residue at position 165 has an amino acid substitution of        M;    -   g) the residue at position 61 has an amino acid substitution of        F;    -   h) the residue at position 170 has an amino acid substitution of        A;    -   i) the residue at position 24 has an amino acid substitution of        F; and    -   j) the residue at position 115 has an amino acid substitution of        L.

In another embodiment, multiple sequence alignment of Pseudomonasfluorescens PF5-ilvC and Bacillus cereus ilvC1 and livC2 and spinachKARI was performed which allowed identification of positions 24, 33, 47,50, 52, 53, 61, 80, 156 and 170 for further mutagenesis. Morespecifically mutants with much lower K_(m) for NADH were obtained. Thesemutations are also based on the Pseudomonas fluorescens, KARI enzyme(SEQ ID NO:17) as a reference sequence wherein the reference sequencecomprises at least one amino acid substitution selected from the groupconsisting of:

-   -   k) the residue at position 24 has an amino acid substitution of        phenylalanine;    -   l) the residue at position 50 has an amino acid substitution of        alanine;    -   m) the residue at position 52 has an amino acid substitution of        aspartic acid;    -   n) the residue at position 53 has an amino acid substitution of        alanine;    -   o) the residue at position 61 has an amino acid substitution of        phenylalanine;    -   p) the residue at position 156 has an amino acid substitution of        valine;    -   q) the residue at position 33 has an amino acid substitution of        leucine;    -   r) the residue at position 47 has an amino acid substitution of        tyrosine;    -   s) the residue at position 80 has an amino acid substitution of        isoleucine;    -   and    -   t) the residue at position 170 has an amino acid substitution of        alanine.

The present invention includes a mutant polypeptide having KARIactivity, said polypeptide having an amino acid sequence selected fromthe group consisting of SEQ ID NO: 24, 25, 26, 27 and 28.

A consensus sequence for the mutant ilvC was generated from the multiplesequence alignment and is provided as SEQ ID NO: 29 which represents allexperimentally verified mutations of the KARI enzyme based on the aminoacid sequence of the KARI enzyme isolated from Pseudomonas fluorescens,(SEQ ID NO:17)

Additionally the present invention describes mutation positionsidentified using a profile Hidden Markov Model (HMM) built based onsequences of 25 functionally verified Class I and Class II KARI enzymes.Profile HMM identified mutation positions 24, 33, 47, 50, 52, 53, 61,80, 115, 156, and 170 (the numbering is based on the sequences ofPseudomonas fluorescens PF5 KARI). Thus, it will be appreciated by theskilled person that mutations at these positions, as well as thosediscussed above that have been experimentally verified will also giverise to KARI enzymes having the ability to bind NADH.

Furthermore, applicants have discovered that the ketol-acidreductoisomerase enzyme has two functionally related domains: one domainaffecting nucleotide specificity and the other domain impacting theK_(M) for the cofactor (FIGS. 11 and 12). To examine whether thischaracteristic could be exploited to engineer the desired KARI mutants(i.e., mutants with high NADH activity (K_(M)<20 μM) and substantiallydecreased NADPH activity (K_(M)>100 μM)), two libraries were created.

One library was a four-site saturation library targeting the NADH orNADPH binding positions, i.e., amino acids at positions 47, 50, 52 and53 (FIG. 11). To build this library, mutants which possessed both NADHand NADPH activities and K_(M)˜10-20 μM for NADH, were selected from agroup consisting of SEQ ID NOs: 28, 67, 68, 69, 70 and 84, as templates.Further saturation mutagenesis generated new mutants (i.e., mutants withSEQ ID NOs: 75-78) that possessed mainly NADH activity with very lowNADPH activity.

The desirable mutants with higher NADH activity, following sitesaturation mutagenesis, comprised the following substitutions:

-   -   u) the residue at position 24 has an amino acid substitution of        phenylalanine;    -   v) the residue at position 50 has an amino acid substitution of        aspartic acid or valine or isoleucine or phenylalanine;    -   w) the residue at position 52 has an amino acid substitution of        tyrosine or aspartic acid;    -   x) the residue at position 53 has an amino acid substitution of        tyrosine or glycine, or argenine, or alanine;    -   y) the residue at position 61 has an amino acid substitution of        phenylalanine;    -   z) the residue at position 156 has an amino acid substitution of        valine;    -   aa) the residue at position 33 has an amino acid substitution of        leucine;    -   bb) the residue at position 47 has an amino acid substitution of        histidine, or proline, or threonine, or glutamic acid; and    -   cc) the residue at position 80 has an amino acid substitution of        isoleucine.

The K_(M) for NADH in the above mutants was still slightly high (e.g.,JB1C6, SEQ ID NO: 74, has K_(M) of 22 μM for NADH). To further improvethe NADH K_(M) of the mutant KARIs, a “domain swapping library”, whichcombined the nucleotide switching mutations and mutations with improvedK_(M) for NADH, was created (FIG. 12). More specifically, the beneficialmutations at positions 47, 50, 52 and 53 obtained in the site saturationexperiment (see Tables 3 and 4), were transferred into mutants thatpossessed K_(M)˜4-40 μM for NADH (SEQ ID NOs:24-28 and 67-70 and 84, seeTables 6 and 7). The resultant new mutants accepted NADH as cofactorwith very low K_(M)˜10 μM and greatly reduced NADPH activity. Examplesof these mutants include: JEA1 (SEQ ID NO: 79), JEG2 (SEQ ID NO: 80),JEG4 (SEQ ID NO: 81), JEA7 (SEQ ID NO: 82) and JED1 (SEQ ID NO: 83).

Following domain swapping experiments, the mutants that possessed verylow K_(M) for NADH had the following substitutions:

-   -   dd) the residue at position 24 has an amino acid substitution of        phenylalanine;    -   ee) the residue at position 50 has an amino acid substitution of        alanine, asparagine, or phenylalanine;    -   ff) the residue at position 52 has an amino acid substitution of        aspartic acid;    -   gg) the residue at position 53 has an amino acid substitution of        alanine;    -   hh) the residue at position 61 has an amino acid substitution of        phenylalanine;    -   ii) the residue at position 156 has an amino acid substitution        of valine;    -   jj) the residue at position 33 has an amino acid substitution of        leucine;    -   kk) the residue at position 47 has an amino acid substitution of        asparagine, proline; and phenylalanine;    -   ll) the residue at position 80 has an amino acid substitution of        isoleucine.

In one embodiment the present method includes a mutant polypeptidehaving KARI activity, said polypeptide having an amino acid sequenceselected from the group consisting of SEQ ID NO: 24,-28, 67-70, and75-98,

In another embodiment the method provides an NADH KARI mutant with aK_(M) for NADH <15 μM.

In a preferred embodiment, the mutant KARI JEA1 (SEQ ID NO: 79) has thefollowing substitutions:

Y24F/C33L/R47P/S50F1T52D/L61F/T80/A156V

In another preferred embodiment, the mutant KARI JEG2 (SEQ ID NO: 80)has the following substitutions:

(Y24F/C33L/R47F/S50A/T52D/V53A/L61F/T80I/A156V)

In another preferred embodiment, the mutant KARI JEG4 (SEQ ID NO: 81),has the following substitutions:

(Y24F/C33L/R47N/S50N/T52D/V53A/L61F/T80I/A156V)

In another preferred embodiment, the mutant KARI JEA7 (SEQ ID NO: 82),has the following substitutions:

(Y24FIC33L/R47P/S50N/T52D/V53A/L61F/T80I/A156V)

In another preferred embodiment, the mutant KARI JED1 (SEQ ID NO: 83)has the following substitutions:

(C33L/R47N/S50N/T52D/V53A/L61F/T80I/A156V)

In another embodiment the method provides an NADH accepting KARI mutantwherein the ratio of NADH/NADPH activity is greater than one. Aconsensus sequence for the mutant ilvC was generated from the multiplesequence alignment and is provided as SEQ ID NO: 29 which represents allexperimentally verified mutations of the KARI enzyme based on the aminoacid sequence of the KARI enzyme isolated from Pseudomonas fluorescens(SEQ ID NO:17).

The Host Strains for KARI Engineering

Two host strains, E. coli TOP10 from Invitrogen and E. coli Bw25113(ΔilvC, an ilvC gene-knockout), were used for making constructsover-expressing the KARI enzyme in this disclosure. In the Bw25113strain, the entire ilvC gene of the E. coli chromosome was replaced by aKanamycin cassette using the Lambda red homology recombinationtechnology described by Kirill et al., (Kirill A. Datsenko and Barry L.Wanner, Proc. Natl. Acad. Sci. USA, 97: 6640-6645, 2000).

Homology Modeling of PF5 KARI with Bound Substrates

The structure of PF5-KARI with bound NADPH, acetolactate and magnesiumions was built based on the crystal structure of P. aeruginosa PAO1-KARI(PDB ID 1NP3, Ahn H. J. et al., J. Mol. Biol., 328: 505-515, 2003) whichhas 92% amino acid sequence homology to PF5 KARI. PAO1-KARI structure isa homo-dodecamer and each dodecamer consists of six homo-dimers withextensive dimer interface. The active site of KARI is located in thisdimer interface. The biological assembly is formed by six homo-dimerspositioned on the edges of a tetrahedron resulting in a highlysymmetrical dodecamer of 23 point group symmetry. For simplicity, onlythe dimeric unit (monomer A and monomer B) was built for the homologymodel of PF5-KARI in this study because the active site is in thehomo-dimer interface.

The model of PF5-KARI dimer was built based on the coordinates ofmonomer A and monomer B of PAO1-KARI and sequence of PF5-KARI usingDeepView/Swiss PDB viewer (Guex, N. and Peitsch, M. C., Electrophoresis,18: 2714-2723, 1997). This model was then imported to program O (Jones,T. A. et al, Acta Crystallogr. A 47: 110-119, 1991) on a SiliconGraphics system for further modification.

The structure of PAO1-KARI has no NADPH, substrate or inhibitor ormagnesium in the active site. Therefore, the spinach KARI structure (PDBID 1yve, Biou V. et al., The EMBO Journal, 16: 3405-3415, 1997.), whichhas magnesium ions, NADPH and inhibitor (N-Hydroxy-N-isopropyloxamate)in the acetolacate binding site, was used to model these molecules inthe active site. The plant KARI has very little sequence homology toeither PF5— or PAO1 KARI (<20% amino acid identity), however thestructures in the active site region of these two KARI enzymes are verysimilar. To overlay the active site of these two KARI structures,commands LSQ_ext, LSQ_improve, LSQ_mol in the program O were used toline up the active site of monomer A of spinach KARI to the monomer A ofPF5 KARI model. The coordinates of NADPH, two magnesium ions and theinhibitor bound in the active site of spinach KARI were extracted andincorporated to molecule A of PF5 KARI. A set of the coordinates ofthese molecules were generated for monomer B of PF5 KARI by applying thetransformation operator from monomer A to monomer B calculated by theprogram.

Because there is no NADPH in the active site of PAO1 KARI crystalstructure, the structures of the phosphate binding loop region in theNADPH binding site (residues 44-45 in PAO1 KARI, 157-170 in spinachKARI) are very different between the two. To model the NADPH bound form,the model of the PF5-KARI phosphate binding loop (44-55) was replaced bythat of 1yve (157-170). Any discrepancy of side chains between these twowas converted to those in the PF5-KARI sequence using the mutate_replacecommand in program 0, and the conformations of the replaced side-chainswere manually adjusted. The entire NADPH/Mg/inhibitor bound dimericPF5-KARI model went through one round of energy minimization usingprogram CNX (ACCELRYS San Diego Calif., Bumger, A. T. and Warren, G. L.,Acta Crystallogr., D 54: 905-921, 1998) after which the inhibitor wasreplaced by the substrate, acetolactate (AL), in the model. Theconformation of AL was manually adjusted to favor hydride transfer of C4of the nicotinamine of NADPH and the substrate. No further energyminimization was performed on this model (coordinates of the modelcreated for this study are attached in a separate word file). Theresidues in the phosphate binding loop and their interactions with NADPHare illustrated in FIG. 3.

Application of a “Profile Hidden Markov Model” for Identification ofResidue Positions Involved in Cofactor Switching in KARI Enzymes

Applicants have developed a method for identifying KARI enzymes and theresidue positions that are involved in cofactor switching from NADPH toNADH. To structurally characterize KARI enzymes, a Profile Hidden MarkovModel (HMM) was prepared as described in Example 5 using amino acidsequences of 25 KARI proteins with experimentally verified function asoutlined in Table 6. These KARIs were from [Pseudomonas fluorescens Pf-5(SEQ ID NO: 17), Sulfolobus solfataricus P2 (SEQ ID NO: 13), Pyrobaculumaerophilum str. IM2 (SEQ ID NO: 14), Natronomonas pharaonis DSM 2160(SEQ ID NO: 30), Bacillus subtilis subsp. subtilis str. 168 (SEQ ID NO:31), Corynebacterium glutamicum ATCC 13032 (SEQ ID NO: 32),Phaeospririlum molischianum (SEQ ID NO: 33), Ralstonia solanacearumGMI1000 (SEQ ID NO: 15), Zymomonas mobilis subsp. mobilis ZM4 (SEQ IDNO: 34), Alkalilimnicola ehrlichei MLHE-1 (SEQ ID NO: 35), Campylobacterlani RM2100 (SEQ ID NO: 36), Marinobacter aquaeolei VT8 (SEQ ID NO: 37),Psychrobacter arcticus 273-4 (SEQ ID NO: 38), Hahella chejuensis KCTC2396 (SEQ ID NO: 39), Thiobacillus denitrificans ATCC 25259 (SEQ ID NO:40), Azotobacter vinelandii AvOP (SEQ ID NO: 41), Pseudomonas syringaepv. syringae B728a (SEQ ID NO: 42), Pseudomonas syringae pv. tomato str.DC3000 (SEQ ID NO: 43), Pseudomonas putida KT2440 (Protein SEQ ID NO:44), Pseudomonas entomophila L48 (SEQ ID NO: 45), Pseudomonas mendocinaymp (SEQ ID NO: 46), Pseudomonas aeruginosa PAO1 (SEQ ID NO: 16),Bacillus cereus ATCC 10987 (SEQ ID NO: 47), Bacillus cereus ATCC 10987(SEQ ID NO: 48), and Spinacia oleracea (SEQ ID NO: 18).

In addition using methods disclosed in this application, sequences ofClass II KARI enzymes such as E. coli (SEQ ID NO: 63-GenBank AccessionNumber P05793), marine gamma Proteobacterium HTCC2207 (SEQ ID NO:64-GenBank Accession Number ZP_(—)01224863.1), Desulfuromonasacetoxidans (SEQ ID NO: 65-GenBank Accession Number ZP_(—)01313517.1)and Pisum sativum (pea) (SEQ ID NO: 66-GenBank Accession Number O82043)could be mentioned.

This Profile HMM for KARIs may be used to identify any KARI relatedproteins. Any protein that matches the Profile HMM with an E value of<10⁻³ using hmmsearch program in the HMMER package is expected to be afunctional KARI, which can be either a Class I and Class II KARI.Sequences matching the Profile HMM given herein are then analyzed forthe location of the 12 positions in Pseudomonas fluorescens Pf-5 thatswitches the cofactor from NADPH to NADH. The eleven nodes, as definedin the section of Profile HMM buiding, in the profile HMM representingthe columns in the alignment which correspond to the eleven co-factorswitching positions in Pseudomonas fluorescens Pf-5 KARI are identifiedas node 24, 33, 47, 50, 52, 53, 61, 80, 115, 156 and 170. The linescorresponding to these nodes in the model file are identified in Table9. One skilled in the art will readily be able to identify these 12positions in the amino acid sequence of a KARI protein from thealignment of the sequence to the profile HMM using hmm search program inHMMER package.

The KARI enzymes identified by this method, include both Class I andClass II KARI enzymes from either microbial or plant natural sources.Any KARI identified by this method may be used for heterologousexpression in microbial cells.

For example each of the KARI encoding nucleic acid fragments describedherein may be used to isolate genes encoding homologous proteins.Isolation of homologous genes using sequence-dependent protocols is wellknown in the art. Examples of sequence-dependent protocols include, butare not limited to: 1) methods of nucleic acid hybridization; 2) methodsof DNA and RNA amplification, as exemplified by various uses of nucleicacid amplification technologies [e.g., polymerase chain reaction (PCR)(Mullis et al., U.S. Pat. No. 4,683,202); ligase chain reaction (LCR)(Tabor, S. et al., Proc. Acad. Sci. USA 82:1074, 1985); or stranddisplacement amplification (SDA) (Walker, et al., Proc. Natl. Acad. Sci.U.S.A., 89: 392, 1992); and 3) methods of library construction andscreening by complementation.

Although the sequence homology between Class I and Class II KARI enzymesis low, the three dimensional structure of both Classes of the enzymes,particularly around the active site and nucleotide binding domains ishighly conserved (Tygai, R., et al., Protein Science, 34: 399-408,2001). The key amino acid residues that make up the substrate bindingpocket are highly conserved between these two Classes even though theymay not align well in a simple sequence comparison. It can therefore beconcluded that the residues affecting cofactor specificity identified inClass I KARI (e.g., positions 24, 33, 47, 50, 52, 53, 61, 80, 115, 156,and 170 of PF5 KARI) can be extended to Class II KARI enzymes.

Isobutanol Biosynthetic Pathways

Carbohydrate utilizing microorganisms employ the Embden-Meyerhof-Pamas(EMP) pathway, the Entner and Doudoroff pathway (EDP) and the pentosephosphate pathway (PPP) as the central, metabolic routes to provideenergy and cellular precursors for growth and maintenance. Thesepathways have in common the intermediate glyceraldehyde-3-phosphate and,ultimately, pyruvate is formed directly or in combination with the EMPpathway. Subsequently, pyruvate is transformed to acetyl-cofactor A(acetyl-CoA) via a variety of means. Acetyl-CoA serves as a keyintermediate, for example, in generating fatty acids, amino acids andsecondary metabolites. The combined reactions of sugar conversion topyruvate produce energy (e.g., adenosine-5′-triphosphate, ATP) andreducing equivalents (e.g., reduced nicotinamide adenine dinucleotide,NADH, and reduced nicotinamide adenine dinucleotide phosphate, NADPH).NADH and NADPH must be recycled to their oxidized forms (NAD⁺ and NADP⁺,respectively). In the presence of inorganic electron acceptors (e.g. O₂,NO₃ ⁻ and SO₄ ²⁻), the reducing equivalents may be used to augment theenergy pool; alternatively, a reduced carbon byproduct may be formed.

There are four potential pathways for production of isobutanol fromcarbohydrate sources with recombinant microorganisms as shown in FIG. 1.All potential pathways for conversion of carbohydrates to isobutanolhave been described in the commonly owned U.S. patent application Ser.No. 11/586,315, which is incorporated herein by reference.

The preferred pathway for conversion of pyruvate to isobutanol consistsof enzymatic steps “a”, “b”, “c”, “d”, and “e” (FIGS. 1A and 1B) andincludes the following substrate to product conversions:

-   -   a) pyruvate to acetolactate, as catalyzed for example by        acetolactate synthase,    -   b) (S)-acetolactate to 2,3-dihydroxyisovalerate, as catalyzed        for example by acetohydroxy acid isomeroreductase,    -   c) 2,3-dihydroxyisovalerate to α-ketoisovalerate, as catalyzed        for example by acetohydroxy acid dehydratase,    -   d) α-ketoisovalerate to isobutyraldehyde, as catalyzed for        example by a branched-chain keto acid decarboxylase, and    -   e) isobutyraldehyde to isobutanol, as catalyzed for example by,        a branched-chain alcohol dehydrogenase.

This pathway combines enzymes involved in well-characterized pathwaysfor valine biosynthesis (pyruvate to α-ketoisovalerate) and valinecatabolism (α-ketoisovalerate to isobutanol). Since many valinebiosynthetic enzymes also catalyze analogous reactions in the isoleucinebiosynthetic pathway, substrate specificity is a major consideration inselecting the gene sources. For this reason, the primary genes ofinterest for the acetolactate synthase enzyme are those from Bacillus(alsS) and Klebsiella (budB). These particular acetolactate synthasesare known to participate in butanediol fermentation in thesemicroorganisms and show increased affinity for pyruvate overketobutyrate (Gollop et al., J. Bacteriol., 172: 3444-3449, 1990); and(Holtzclaw et al., J. Bacteriol., 121: 917-922, 1975). The second andthird pathway steps are catalyzed by acetohydroxy acid reductoisomeraseand dehydratase, respectively. These enzymes have been characterizedfrom a number of sources, such as for example, E. coli (Chunduru et al.,Biochemistry, 28: 486-493, 1989); and (Flint et al., J. Biol. Chem.,268: 14732-14742, 1993). The final two steps of the preferred isobutanolpathway are known to occur in yeast, which can use valine as a nitrogensource and, in the process, secrete isobutanol. α-Ketoiso-valerate canbe converted to isobutyraldehyde by a number of keto acid decarboxylaseenzymes, such as for example pyruvate decarboxylase. To preventmisdirection of pyruvate away from isobutanol production, adecarboxylase with decreased affinity for pyruvate is desired. So far,there are two such enzymes known in the art (Smit et al., Appl. Environ.Microbiol., 71: 303-311, 2005); and (de la Plaza et al., FEMS Microbiol.Lett., 238: 367-374, 2004). Both enzymes are from strains of Lactococcuslactis and have a 50-200-fold preference for ketoisovalerate overpyruvate. Finally, a number of aldehyde reductases have been identifiedin yeast, many with overlapping substrate specificity. Those known toprefer branched-chain substrates over acetaldehyde include, but are notlimited to, alcohol dehydrogenase VI (ADH6) and Ypr1p (Larroy et al.,Biochem. J., 361: 163-172, 2002); and (Ford et al., Yeast, 19:1087-1096, 2002), both of which use NADPH as electron donor. AnNADPH-dependent reductase, YqhD, active with branched-chain substrateshas also been recently identified in E. coli (Sulzenbacher et al., J.Mol. Biol., 342: 489-502, 2004).

Two of the other potential pathways for isobutanol production alsocontain the initial three steps of “a”, “b” and “c” (FIG. 1A). Onepathway consists of enzymatic steps “a”, “b”, “c”, “f”, “g”, “e” (FIGS.1A and 1B). Step “f” containing a “branched-chain keto aciddehydrogenase (EC1.2.4.4). Step “g” containing an “acylating aldehydedehydrogenase” (EC1.2.1.10) and 1.2.1.57 in addition to step “e”containing the “branched chain alcohol dehydrogenase”. The otherpotential pathway consists of steps “a”, “b”, “c”, “h”, “i”, “j”, “e”(FIGS. 1A and 1B). The term “transaminase” (step “h”) EC numbers2.6.1.42 and 2.6.1.66. Step “h” consists of either a “valinedehydrogenase” (EC1.4.1.8 and EC1.4.1.9) or step “i”, a “valinedecarboxylase” with an EC number 4.1.1.14. Finally step “j” will use an“omega transaminase” (EC2.6.1.18) to generate isobutyraldehyde whichwill be reduced by step “e” to produce isobutanol. All potentialpathways for conversion of pyruvate to isobutanol are depicted in FIGS.1A and 1B.

Additionally, a number of microorganisms are known to produce butyrateand/or butanol via a butyryl-CoA intermediate (Durre, et al., FEMSMicrobiol. Rev., 17: 251-262, 1995); and (Abbad-Andaloussi et al.,Microbiology, 142: 1149-1158, 1996). Therefore isobutanol production inthese microorganisms will take place using steps “k”, “g” and “e” shownin FIG. 1B. Step “k” will use an “isobutyryl-CoA mutase” (EC5.4.99.13).The nest step will involve using the “acylating aldehyde dehydrogenase”(EC 1.2.1.10 and EC1.2.1.57) to produce isobutyraldehyde followed byenzymatic step “e” to produce isobutanol. All these pathways are fullydescribed in the commonly owned patent application Ser. No. 11/586,315,herein incorporated by reference.

Thus, in providing multiple recombinant pathways from pyruvate toisobutanol, there exist a number of choices to fulfill the individualconversion steps, and the person of skill in the art will be able to usepublicly available sequences to construct the relevant pathways.

Microbial Hosts for Isobutanol Production

Microbial hosts for isobutanol production may be selected from bacteria,cyanobacteria, filamentous fungi and yeasts. The microbial host used forisobutanol production should be tolerant to isobutanol so that the yieldis not limited by butanol toxicity. Microbes that are metabolicallyactive at high titer levels of isobutanol are not well known in the art.Although butanol-tolerant mutants have been isolated from solventogenicClostridia, little information is available concerning the butanoltolerance of other potentially useful bacterial strains. Most of thestudies on the comparison of alcohol tolerance in bacteria suggest thatbutanol is more toxic than ethanol (de Cavalho, et al., Microsc. Res.Tech., 64: 215-22, 2004) and (Kabelitz, et al., FEMS Microbiol. Lett.,220: 223-227, 2003, Tomas, et al., J. Bacteriol., 186: 2006-2018, 2004)report that the yield of 1-butanol during fermentation in Clostridiumacetobutylicum may be limited by 1-butanol toxicity. The primary effectof 1-butanol on Clostridium acetobutylicum is disruption of membranefunctions (Hermann et al., Appl. Environ. Microbiol., 50: 1238-1243,1985).

The microbial hosts selected for the production of isobutanol should betolerant to isobutanol and should be able to convert carbohydrates toisobutanol. The criteria for selection of suitable microbial hostsinclude the following: intrinsic tolerance to isobutanol, high rate ofglucose utilization, availability of genetic tools for genemanipulation, and the ability to generate stable chromosomalalterations.

Suitable host strains with a tolerance for isobutanol may be identifiedby screening based on the intrinsic tolerance of the strain. Theintrinsic tolerance of microbes to isobutanol may be measured bydetermining the concentration of isobutanol that is responsible for 50%inhibition of the growth rate (IC₅₀) when grown in a minimal medium. TheIC₅₀ values may be determined using methods known in the art. Forexample, the microbes of interest may be grown in the presence ofvarious amounts of isobutanol and the growth rate monitored by measuringthe optical density at 600 nanometers. The doubling time may becalculated from the logarithmic part of the growth curve and used as ameasure of the growth rate. The concentration of isobutanol thatproduces 50% inhibition of growth may be determined from a graph of thepercent inhibition of growth versus the isobutanol concentration.Preferably, the host strain should have an IC₅₀ for isobutanol ofgreater than about 0.5%.

The microbial host for isobutanol production should also utilize glucoseat a high rate. Most microbes are capable of metabolizing carbohydrates.However, certain environmental microbes cannot metabolize carbohydratesto high efficiency, and therefore would not be suitable hosts.

The ability to genetically modify the host is essential for theproduction of any recombinant microorganism. The mode of gene transfertechnology may be by electroporation, conjugation, transduction ornatural transformation. A broad range of host conjugative plasmids anddrug resistance markers are available. The cloning vectors are tailoredto the host microorganisms based on the nature of antibiotic resistancemarkers that can function in that host.

The microbial host also has to be manipulated in order to inactivatecompeting pathways for carbon flow by deleting various genes. Thisrequires the availability of either transposons to direct inactivationor chromosomal integration vectors. Additionally, the production hostshould be amenable to chemical mutagenesis so that mutations to improveintrinsic isobutanol tolerance may be obtained.

Based on the criteria described above, suitable microbial hosts for theproduction of isobutanol include, but are not limited to, members of thegenera Clostridium, Zymomonas, Escherichia, Salmonella, Rhodococcus,Pseudomonas, Bacillus, Vibrio, Lactobacillus, Enterococcus, Alcaligenes,Klebsiella, Paenibacillus, Arthrobacter, Corynebacterium,Brevibacterium, Pichia, Candida, Hansenula and Saccharomyces. Preferredhosts include: Escherichia coli, Alcaligenes eutrophus, Bacilluslicheniformis, Paenibacillus macerans, Rhodococcus erythropolis,Pseudomonas putida, Lactobacillus plantarum, Enterococcus faecium,Enterococcus gallinarium, Enterococcus faecalis, Bacillus subtilis andSaccharomyces cerevisiae.

Construction of Production Host

Recombinant microorganisms containing the necessary genes that willencode the enzymatic pathway for the conversion of a fermentable carbonsubstrate to isobutanol may be constructed using techniques well knownin the art. In the present invention, genes encoding the enzymes of oneof the isobutanol biosynthetic pathways of the invention, for example,acetolactate synthase, acetohydroxy acid isomeroreductase, acetohydroxyacid dehydratase, branched-chain α-keto acid decarboxylase, andbranched-chain alcohol dehydrogenase, may be isolated from varioussources, as described above.

Methods of obtaining desired genes from a bacterial genome are commonand well known in the art of molecular biology. For example, if thesequence of the gene is known, suitable genomic libraries may be createdby restriction endonuclease digestion and may be screened with probescomplementary to the desired gene sequence. Once the sequence isisolated, the DNA may be amplified using standard primer-directedamplification methods such as polymerase chain reaction (U.S. Pat. No.4,683,202) to obtain amounts of DNA suitable for transformation usingappropriate vectors. Tools for codon optimization for expression in aheterologous host are readily available. Some tools for codonoptimization are available based on the GC content of the hostmicroorganism.

Once the relevant pathway genes are identified and isolated they may betransformed into suitable expression hosts by means well known in theart. Vectors or cassettes useful for the transformation of a variety ofhost cells are common and commercially available from companies such asEPICENTRE® (Madison, Wis.), Invitrogen Corp. (Carlsbad, Calif.),Stratagene (La Jolla, Calif.), and New England Biolabs, Inc. (Beverly,Mass.). Typically the vector or cassette contains sequences directingtranscription and translation of the relevant gene, a selectable marker,and sequences allowing autonomous replication or chromosomalintegration. Suitable vectors comprise a region 5′ of the gene whichharbors transcriptional initiation controls and a region 3′ of the DNAfragment which controls transcriptional termination. Both controlregions may be derived from genes homologous to the transformed hostcell, although it is to be understood that such control regions may alsobe derived from genes that are not native to the specific species chosenas a production host.

Initiation control regions or promoters, which are useful to driveexpression of the relevant pathway coding regions in the desired hostcell are numerous and familiar to those skilled in the art. Virtuallyany promoter capable of driving these genetic elements is suitable forthe present invention including, but not limited to, CYC1, HIS3, GAL1,GAL10, ADH1, PGK, PHO5, GAPDH, ADC1, TRP1, URA3, LEU2, ENO, TPI (usefulfor expression in Saccharomyces); AOX1 (useful for expression inPichia); and lac, ara, tet, tip, IP_(L), IP_(R), T7, tac, and trc(useful for expression in Escherichia coli, Alcaligenes, andPseudomonas) as well as the amy, apr, npr promoters and various phagepromoters useful for expression in Bacillus subtilis, Bacilluslicheniformis, and Paenibacillus macerans.

Termination control regions may also be derived from various genesnative to the preferred hosts. Optionally, a termination site may beunnecessary, however, it is most preferred if included.

Certain vectors are capable of replicating in a broad range of hostbacteria and can be transferred by conjugation. The complete andannotated sequence of pRK404 and three related vectors-pRK437, pRK442,and pRK442(H) are available. These derivatives have proven to bevaluable tools for genetic manipulation in Gram-negative bacteria (Scottet al., Plasmid, 50: 74-79, 2003). Several plasmid derivatives ofbroad-host-range Inc P4 plasmid RSF1010 are also available withpromoters that can function in a range of Gram-negative bacteria.Plasmid pAYC36 and pAYC37, have active promoters along with multiplecloning sites to allow for the heterologous gene expression inGram-negative bacteria.

Chromosomal gene replacement tools are also widely available. Forexample, a thermosensitive variant of the broad-host-range repliconpWV101 has been modified to construct a plasmid pVE6002 which can beused to effect gene replacement in a range of Gram-positive bacteria(Maguin et al., J. Bacteriol., 174: 5633-5638, 1992). Additionally, invitro transposomes are available to create random mutations in a varietyof genomes from commercial sources such as EPICENTRE®.

The expression of an isobutanol biosynthetic pathway in variouspreferred microbial hosts is described in more detail below.

Expression of an Isobutanol Biosynthetic Pathway in E. Coli

Vectors or cassettes useful for the transformation of E. coli are commonand commercially available from the companies listed above. For example,the genes of an isobutanol biosynthetic pathway may be isolated fromvarious sources, cloned into a modified pUC19 vector and transformedinto E. coli NM522.

Expression of an Isobutanol Biosynthetic Pathway in Rhodococcuserythropolis

A series of E. coli-Rhodococcus shuttle vectors are available forexpression in R. erythropolis, including, but not limited to, pRhBR17and pDA71 (Kostichka et al., Appl. Microbiol. Biotechnol., 62: 61-68,2003). Additionally, a series of promoters are available forheterologous gene expression in R. erythropolis (Nakashima et al., Appl.Environ. Microbiol., 70: 5557-5568, 2004 and Tao et al., Appl.Microbiol. Biotechnol., 68: 346-354, 2005). Targeted gene disruption ofchromosomal genes in R. erythropolis may be created using the methoddescribed by Tao et al., supra, and Brans et al. (Appl. Environ.Microbiol., 66: 2029-2036, 2000).

The heterologous genes required for the production of isobutanol, asdescribed above, may be cloned initially in pDA71 or pRhBR71 andtransformed into E. coli. The vectors may then be transformed into R.erythropolis by electroporation, as described by Kostichka et al.,supra. The recombinants may be grown in synthetic medium containingglucose and the production of isobutanol can be followed using methodsknown in the art.

Expression of an Isobutanol Biosynthetic Dathway in B. Subtilis

Methods for gene expression and creation of mutations in B. subtilis arealso well known in the art. For example, the genes of an isobutanolbiosynthetic pathway may be isolated from various sources, cloned into amodified pUC19 vector and transformed into Bacillus subtilis BE1010.Additionally, the five genes of an isobutanol biosynthetic pathway canbe split into two operons for expression. The three genes of the pathway(bubB, ilvD, and kivD) can be integrated into the chromosome of Bacillussubtilis BE1010 (Payne, et al., J. Bacteriol., 173, 2278-2282, 1991).The remaining two genes (ilvC and bdhB) can be cloned into an expressionvector and transformed into the Bacillus strain carrying the integratedisobutanol genes

Expression of an Isobutanol Biosynthetic Dathway in B. Licheniformis

Most of the plasmids and shuttle vectors that replicate in B. subtilismay be used to transform B. licheniformis by either protoplasttransformation or electroporation. The genes required for the productionof isobutanol may be cloned in plasmids pBE20 or pBE60 derivatives(Nagarajan et al., Gene, 114: 121-126, 1992). Methods to transform B.licheniformis are known in the art (Fleming et al. Appl. Environ.Microbiol., 61: 3775-3780, 1995). The plasmids constructed forexpression in B. subtilis may be transformed into B. licheniformis toproduce a recombinant microbial host that produces isobutanol.

Expression of an Isobutanol Biosynthetic Pathway in Paenibacillusmacerans

Plasmids may be constructed as described above for expression in B.subtilis and used to transform Paenibacillus macerans by protoplasttransformation to produce a recombinant microbial host that producesisobutanol.

Expression of the Isobutanol Biosynthetic Pathway in Alcalienes(Ralstonia) eutrophus

Methods for gene expression and creation of mutations in Alcaligeneseutrophus are known in the art (Taghavi et al., Appl. Environ.Microbiol., 60: 3585-3591, 1994). The genes for an isobutanolbiosynthetic pathway may be cloned in any of the broad host rangevectors described above, and electroporated to generate recombinantsthat produce isobutanol. The poly(hydroxybutyrate) pathway inAlcaligenes has been described in detail, a variety of genetictechniques to modify the Alcaligenes eutrophus genome is known, andthose tools can be applied for engineering an isobutanol biosyntheticpathway.

Expression of an Isobutanol Biosynthetic Pathway in Pseudomonas putida

Methods for gene expression in Pseudomonas putida are known in the art(see for example Ben-Bassat et al., U.S. Pat. No. 6,586,229, which isincorporated herein by reference). The butanol pathway genes may beinserted into pPCU18 and this ligated DNA may be electroporated intoelectrocompetent Pseudomonas putida DOT-T1 C5aAR1 cells to generaterecombinants that produce isobutanol.

Expression of an Isobutanol Biosynthetic Pathway in Saccharomycescerevisiae

Methods for gene expression in Saccharomyces cerevisiae are known in theart (e.g., Methods in Enzymology, Volume 194, Guide to Yeast Geneticsand Molecular and Cell Biology, Part A, 2004, Christine Guthrie andGerald R. Fink, eds., Elsevier Academic Press, San Diego, Calif.).Expression of genes in yeast typically requires a promoter, followed bythe gene of interest, and a transcriptional terminator. A number ofyeast promoters can be used in constructing expression cassettes forgenes encoding an isobutanol biosynthetic pathway, including, but notlimited to constitutive promoters FBA, GPD, ADH1, and GPM, and theinducible promoters GAL1, GAL10, and CUP1. Suitable transcriptionalterminators include, but are not limited to FBAt, GPDt, GPMt, ERG10t,GAL1t, CYC1, and ADH1. For example, suitable promoters, transcriptionalterminators, and the genes of an isobutanol biosynthetic pathway may becloned into E. coli-yeast shuttle vectors.

Expression of an Isobutanol Biosynthetic Pathway in Lactobacillusplantarum

The Lactobacillus genus belongs to the Lactobacillales family and manyplasmids and vectors used in the transformation of Bacillus subtilis andStreptococcus may be used for lactobacillus. Non-limiting examples ofsuitable vectors include pAM31 and derivatives thereof (Renault et al.,Gene 183:175-182, 1996); and (O'Sullivan et al., Gene, 137: 227-231,1993); pMBB1 and pHW800, a derivative of pMBB1 (Wyckoff et al., Appl.Environ. Microbiol., 62: 1481-1486, 1996); pMG1, a conjugative plasmid(Tanimoto et al., J. Bacteriol., 184: 5800-5804, 2002); pNZ9520(Kleerebezem et al., Appl. Environ. Microbiol., 63: 4581-4584, 1997);pAM401 (Fujimoto et al., Appl. Environ. Microbiol., 67: 1262-1267,2001); and pAT392 (Arthur et al., Antimicrob. Agents Chemother., 38:1899-1903, 1994). Several plasmids from Lactobacillus plantarum havealso been reported (van Kranenburg R, et al. Appl. Environ. Microbiol.,71: 1223-1230, 2005).

Expression of an Isobutanol Biosynthetic Pathway in Various Enterococcusspecies (E. faecium, E. aallinarium, and E. faecalis)

The Enterococcus genus belongs to the Lactobacillales family and manyplasmids and vectors used in the transformation of Lactobacilli, Bacilliand Streptococci species may be used for Enterococcus species.Non-limiting examples of suitable vectors include pAMβ1 and derivativesthereof (Renault et al., Gene, 183: 175-182, 1996); and (O'Sullivan etal., Gene, 137: 227-231, 1993); pMBB1 and pHW800, a derivative of pMBB1(Wyckoff et al. Appl. Environ. Microbiol., 62: 1481-1486, 1996); pMG1, aconjugative plasmid (Tanimoto et al., J. Bacteriol., 184: 5800-5804,2002); pNZ9520 (Kleerebezem et al., Appl. Environ. Microbiol., 63:4581-4584, 1997); pAM401 (Fujimoto et al., Appl. Environ. Microbiol.,67: 1262-1267, 2001); and pAT392 (Arthur et al., Antimicrob. AgentsChemother., 38: 1899-1903, 1994). Expression vectors for E. faecalisusing the nisA gene from Lactococcus may also be used (Eichenbaum etal., Appl. Environ. Microbiol., 64: 2763-2769, 1998). Additionally,vectors for gene replacement in the E. faecium chromosome may be used(Nallaapareddy et al., Appl. Environ. Microbiol., 72: 334-345, 2006).

Fermentation Media

Fermentation media in the present invention must contain suitable carbonsubstrates. Suitable substrates may include but are not limited tomonosaccharides such as glucose and fructose, oligosaccharides such aslactose or sucrose, polysaccharides such as starch or cellulose ormixtures thereof and unpurified mixtures from renewable feedstocks suchas cheese whey permeate, comsteep liquor, sugar beet molasses, andbarley malt. Additionally the carbon substrate may also be one-carbonsubstrates such as carbon dioxide, or methanol for which metabolicconversion into key biochemical intermediates has been demonstrated. Inaddition to one and two carbon substrates methylotrophic microorganismsare also known to utilize a number of other carbon containing compoundssuch as methylamine, glucosamine and a variety of amino acids formetabolic activity. For example, methylotrophic yeast are known toutilize the carbon from methylamine to form trehalose or glycerol(Bellion et al., Microb. Growth C1 Compd., [Int. Symp.], 7th (1993),415-32. (eds): Murrell, J. Collin; Kelly, Don P. Publisher: Intercept,Andover, UK). Similarly, various species of Candida will metabolizealanine or oleic acid (Sulter et al., Arch. Microbiol., 153: 485-489,1990). Hence it is contemplated that the source of carbon utilized inthe present invention may encompass a wide variety of carbon containingsubstrates and will only be limited by the choice of microorganism.

Although it is contemplated that all of the above mentioned carbonsubstrates and mixtures thereof are suitable in the present invention,preferred carbon substrates are glucose, fructose, and sucrose.

In addition to an appropriate carbon source, fermentation media mustcontain suitable minerals, salts, cofactors, buffers and othercomponents, known to those skilled in the art, suitable for growth ofthe cultures and promotion of the enzymatic pathway necessary forisobutanol production.

Culture Conditions

Typically cells are grown at a temperature in the range of about 25° C.to about 40° C. in an appropriate medium. Suitable growth media in thepresent invention are common commercially prepared media such as LuriaBertani (LB) broth, Sabouraud Dextrose (SD) broth or Yeast Medium (YM)broth. Other defined or synthetic growth media may also be used, and theappropriate medium for growth of the particular microorganism will beknown by one skilled in the art of microbiology or fermentation science.The use of agents known to modulate catabolite repression directly orindirectly, e.g., cyclic adenosine 2′,3′-monophosphate (cAMP), may alsobe incorporated into the fermentation medium.

Suitable pH ranges for the fermentation are between pH 5.0 to pH 9.0,where pH 6.0 to pH 8.0 is preferred for the initial condition.

Fermentations may be performed under aerobic or anaerobic conditions,where anaerobic or microaerobic conditions are preferred.

Industrial Batch and Continuous Fermentations

The present process employs a batch method of fermentation. A classicalbatch fermentation is a closed system where the composition of themedium is set at the beginning of the fermentation and not subject toartificial alterations during the fermentation. Thus, at the beginningof the fermentation the medium is inoculated with the desiredmicroorganism or microorganisms, and fermentation is permitted to occurwithout adding anything to the system. Typically, however, a “batch”fermentation is batch with respect to the addition of carbon source andattempts are often made at controlling factors such as pH and oxygenconcentration. In batch systems the metabolite and biomass compositionsof the system change constantly up to the time the fermentation isstopped. Within batch cultures cells moderate through a static lag phaseto a high growth log phase and finally to a stationary phase wheregrowth rate is diminished or halted. If untreated, cells in thestationary phase will eventually die. Cells in log phase generally areresponsible for the bulk of production of end product or intermediate.

A variation on the standard batch system is the Fed-Batch system.Fed-Batch fermentation processes are also suitable in the presentinvention and comprise a typical batch system with the exception thatthe substrate is added in increments as the fermentation progresses.Fed-Batch systems are useful when catabolite repression is apt toinhibit the metabolism of the cells and where it is desirable to havelimited amounts of substrate in the medium. Measurement of the actualsubstrate concentration in Fed-Batch systems is difficult and istherefore estimated on the basis of the changes of measurable factorssuch as pH, dissolved oxygen and the partial pressure of waste gasessuch as CO₂. Batch and Fed-Batch fermentations are common and well knownin the art and examples may be found in Thomas D. Brock inBiotechnology: A Textbook of Industrial Microbiology, Second Edition(1989) Sinauer Associates, Inc., Sunderland, Mass., or Deshpande, Mukund(Appl. Biochem. Biotechnol., 36: 227, 1992), herein incorporated byreference.

Although the present invention is performed in batch mode it iscontemplated that the method would be adaptable to continuousfermentation methods. Continuous fermentation is an open system where adefined fermentation medium is added continuously to a bioreactor and anequal amount of conditioned medium is removed simultaneously forprocessing. Continuous fermentation generally maintains the cultures ata constant high density where cells are primarily in log phase growth.

Continuous fermentation allows for modulation of one factor or anynumber of factors that affect cell growth or end product concentration.For example, one method will maintain a limiting nutrient such as thecarbon source or nitrogen level at a fixed rate and allow all otherparameters to moderate. In other systems a number of factors affectinggrowth may be altered continuously while the cell concentration,measured by medium turbidity, is kept constant. Continuous systemsstrive to maintain steady state growth conditions and thus the cell lossdue to the medium being drawn off must be balanced against the cellgrowth rate in the fermentation. Methods of modulating nutrients andgrowth factors for continuous fermentation processes as well astechniques for maximizing the rate of product formation are well knownin the art of industrial microbiology and a variety of methods aredetailed by Brock, supra.

It is contemplated that the present invention may be practiced usingeither batch, fed-batch or continuous processes and that any known modeof fermentation would be suitable. Additionally, it is contemplated thatcells may be immobilized on a substrate as whole cell catalysts andsubjected to fermentation conditions for isobutanol production.

Methods for Isobutanol Isolation from the Fermentation Medium

The biologically produced isobutanol may be isolated from thefermentation medium using methods known in the art forAcetone-butanol-ethanol (ABE) fermentations (see for example, Durre,Appl. Microbiol. Biotechnol. 49: 639-648, 1998), and (Groot et al.,Process. Biochem. 27: 61-75, 1992 and references therein). For example,solids may be removed from the fermentation medium by centrifugation,filtration, decantation and isobutanol may be isolated from thefermentation medium using methods such as distillation, azeotropicdistillation, liquid-liquid extraction, adsorption, gas stripping,membrane evaporation, or pervaporation.

EXAMPLES

The present invention is further defined in the following Examples. Itshould be understood that these Examples, while indicating preferredembodiments of the invention, are given by way of illustration only.From the above discussion and these Examples, one skilled in the art canascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various uses andconditions.

General Methods:

Standard recombinant DNA and molecular cloning techniques used in theExamples are well known in the art and are described by Sambrook, J.,Fritsch, E. F. and Maniatis, T., Molecular Cloning: A Laboratory Manual,Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989, byT. J. Silhavy, M. L. Bennan, and L. W. Enquist, Experiments with GeneFusions, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., 1984,and by Ausubel, F. M. et al., Current Protocols in Molecular Biology,Greene Publishing Assoc. and Wiley-Interscience, N.Y., 1987. Materialsand methods suitable for the maintenance and growth of bacterialcultures are also well known in the art. Techniques suitable for use inthe following Examples may be found in Manual of Methods for GeneralBacteriology. Phillipp Gerhardt, R. G. E. Murray, Ralph N. Costilow,Eugene W. Nester, Willis A. Wood, Noel R. Krieg and G. Briggs Phillips,eds., American Society for Microbiology, Washington, D.C., 1994, or byThomas D. Brock in Biotechnology: A Textbook of Industrial Microbiology,Second Edition, Sinauer Associates, Inc., Sunderland, Mass., 1989. Allreagents, restriction enzymes and materials used for the growth andmaintenance of bacterial cells were obtained from Aldrich Chemicals(Milwaukee, Wis.), BD Diagnostic Systems (Sparks, Md.), LifeTechnologies (Rockville, Md.), or Sigma Chemical Company (St. Louis,Mo.), unless otherwise specified.

The meaning of abbreviations used is as follows: “A” means Angstrom,“min” means minute(s), “h” means hour(s), “μl” means microliter(s),“ng/μl” means nano gram per microliter, “pmol/μl” means pico mole permicroliter, “ml” means milliliter(s), “L” means liter(s), “g/L” meangram per liter, “ng” means nano gram, “sec” means second(s), “ml/min”means milliliter per minute(s), “w/v” means weight per volume, “v/v”means volume per volume, “nm” means nanometer(s), “mm” meansmillimeter(s), “cm” means centimeter(s), “mM” means millimolar, “M”means molar, “mmol” means millimole(s), “μmole” means micromole(s), “g”means gram(s), “μg” means microgram(s), “mg” means milligram(s), “g”means the gravitation constant, “rpm” means revolutions per minute,“HPLC” means high performance liquid chromatography, “MS” means massspectrometry, “HPLC/MS” means high performance liquidchromatography/mass spectrometry, “EDTA” meansethylendiamine-tetraacetic acid, “dNTP” means deoxynucleotidetriphosphate, “° C.” means degrees Celsius, and “V” means voltage.

The oligonucleotide primers used in the following Examples have beendescribed herein (see Table 1).

High Throughput Screening Assay of Gene Libraries

High throughput screening of the gene libraries of mutant KARI enzymeswas performed as described herein: 10× freezing medium containing 554.4g/L glycerol, 68 mM of (NH₄)₂SO₄, 4 mM MgSO₄, 17 mM sodium citrate, 132mM KH₂PO₄, 36 mM K₂HPO₄ was prepared with molecular pure water andfilter-sterilized. Freezing medium was prepared by diluting the 10×freezing medium with the LB medium. An aliquot (200 μl) of the freezingmedium was used for each well of the 96-well archive plates (cat #3370,Corning Inc. Corning, N.Y.).

Clones from the LB agar plates were selected and inoculated into the96-well archive plates containing the freezing medium and grownovernight at 37° C. without shaking. The archive plates were then storedat −80° C. E. coli strain Bw25113 transformed with pBAD-H isB(Invitrogen) was always used as the negative control. For libraries C,E, F and G, mutant T52D of (PF5-ilvC) was used as the positive control.The mutant T52D was a mutant of PF5-ilvC in which the threonine atposition 52 was changed to aspartic acid. For library H, mutant C3B11(R47F/S50A/T52D/v53W of PF5-ilvC) was used as the positive control.

Clones from archive plates were inoculated into the 96-deep well plates.Each well contained 3.0 μl of cells from thawed archive plates, 300 μlof the LB medium containing 100 μg/ml ampicillin and 0.02% (w/v)arabinose as the inducer. Cells were the grown overnight at 37° C. with80% humidity while shaking (900 rpm), harvested by centrifugation (4000rpm, 5 min at 25° C.). (Eppendorf centrifuge, Brinkmann Instruments,Inc. Westbury, N.Y.) and the cell pellet was stored at −20° C. for lateranalysis.

The assay substrate, (R,S)-acetolactate, was synthesized as described byAulabaugh and Schloss (Aulabaugh and Schloss, Biochemistry, 29:2824-2830, 1990): 1.0 g of 2-acetoxy-2-methyl-3-oxobutyric acid ethylester (Aldrich, Milwaukee, Wis.) was mixed with 10 ml NaOH (1.0 M) andstirred at room temperature. When the solution's pH became neutral,additional NaOH was slowly added until pH ˜8.0 was maintained. All otherchemicals used in the assay were purchased from Sigma.

The enzymatic conversion of acetolactate to α,β-dihydroxyisovalerate byKARI was followed by measuring the disappearance of the cofactor, NADPHor NADH, from the reaction at 340 nm using a plate reader (MolecularDevice, Sunnyvale, Calif.). The activity was calculated using the molarextinction coefficient of 6220 M⁻¹ cm⁻¹ for either NADPH or NADH. Thestock solutions used were: K₂HPO₄ (0.2 M); KH₂PO₄ (0.2 M); EDTA (0.5 M);MgCl₂ (1.0 M); NADPH (2.0 mM); NADH (2.0 mM) and acetolactate (45 mM).The 100 ml reaction buffer mix stock containing: 4.8 ml K₂HPO₄, 0.2 mlKH₂PO₄, 4.0 ml MgCl₂, 0.1 ml EDTA and 90.9 ml water was prepared.

Frozen cell pellet in deep-well plates and BugBuster were warmed up atroom temperature for 30 min at the same time. Each well of 96-well assayplates was filled with 120 μl of the reaction buffer and 20 μl of NADH(2.0 mM), 1501 of BugBuster was added to each well after 30 min warm-upand cells were suspended using Genmate (Tecan Systems Inc. San Jose,Calif.) by pipetting the cell suspension up and down (×5). The plateswere incubated at room temperature for 20 min and then heated at 60° C.for 10 min. The cell debris and protein precipitates were removed bycentrifugation at 4,000 rpm for 5 min at 25° C. An aliquot (50 μl) ofthe supernatant was transferred into each well of 96-well assay plates,the solution was mixed and the bubbles were removed by centrifugation at4,000 rpm at 25° C. for 1 min. Absorbance at 340 nm was recorded asbackground, 20 μl of acetolactate (4.5 mM, diluted with the reactionbuffer) was added to each well and mixed with shaking by the platereader. Absorbance at 340 nm was recoded at 0, and 60 minutes aftersubstrate addition. The difference in absorbance (before and aftersubstrate addition) was used to determine the activity of the mutants.Mutants with higher KARI activity compared to the wild type wereselected for re-screening.

About 5,000 clones were screened for library C and 360 top performerswere selected for re-screen. About 92 clones were screened for library Eand 16 top performers were selected for re-screening. About 92 cloneswere screened for library F and 8 top performers were selected forre-screening. About 92 clones were screened for library G and 20 topperformers were selected for re-screening. About 8,000 clones werescreened for library H and 62 top performers were selected forre-screening. The re-screening was described below as secondary assay.

Secondary Assay of Active Mutants

Cells containing pBad-ilvC and its mutants identified by high throughputscreening were grown overnight, at 37° C., in 3.0 ml of the LB mediumcontaining 100 pg/ml ampicillin and 0.02% (w/v) arabinose as the inducerwhile shaking at 250 rpm. The cells were then harvested bycentrifugation at 18,000×g for 1 min at room temperature (Sigmamicro-centrifuge model 1-15, Laurel, Md.). The cell pellets werere-suspended in 3001 of BugBuster Master Mix (EMD Chemicals). Thereaction mixture was first incubated at room temperature for 20 min andthen heated at 60° C. for 10 min. The cell debris and proteinprecipitate were removed by centrifugation at 18,000×g for 5 min at roomtemperature.

The reaction buffer (120 μl) prepared as described above was mixed witheither NADH or NADPH (20 μl) stock and cell extract (20 μl) in each wellof a 96-well assay plate. The absorbance at 340 nm at 25° C. wasrecorded as background. Then 20 μl of acetolactate (4.5 mM, diluted withreaction buffer) was added each well and mixed with shaking by the platereader. The absorbance at 340 nm at 0 min, 2 min and 5 min after addingacetolactate was recorded. The absorbance difference before and afteradding substrate was used to determine the activity of the mutants. Themutants with high activity were selected for sequencing.

Five top performers from “Library C” were identified and sequenced (FIG.4). The best performer was mutant R47F/S50A/T52D/V53W, which completelyreversed the nucleotide specificity. The best performers from “LibrariesE, F and G” were R47P, S50D and T52D respectively (FIG. 5). For “LibraryH”, 5 top performers were identified and sequenced (FIG. 6) and the bestperformer was R47P/S50G/T52D, which also completely reversed thenucleotide specificity. Enzymes containing activities higher than thebackground were considered positive.

KARI Enzyme Assay

KARI enzyme activity can be routinely measured by NADH or NADPHoxidation as described above, however to measure formation of the2,3-dihydroxyisovalerate product directly, analysis of the reaction wasperformed using HPLC/MS.

Protein concentration of crude cell extract from Bugbuster lysed cells(as described above) was measured using the BioRad protein assay reagent(BioRad Laboratories, Inc., Hercules, Calif. 94547). A total of 0.5micrograms of crude extract protein was added to a reaction bufferconsisting of 100 mM HEPES-KOH, pH 7.5, 10 mM MgCl₂, 1 mMglucose-6-phosphate (Sigma-Aldrich), 0.2 Units of Leuconostocmesenteroides glucose-6-phosphate dehydrogenase (Sigma-Aldrich), andvarious concentrations of NADH or NADPH, to a volume of 96 μL. Thereaction was initiated by the addition of 4 μL of acetolactate to afinal concentration of 4 mM and a final volume of 100 μL. After timedincubations at 30° C., typically between 2 and 15 min, the reaction wasquenched by the addition of 10 μL of 0.5 M EDTA, pH 8.0 (LifeTechnologies, Grand Island, N.Y. 14072). To measure the K_(M) of NADH,the concentrations used were 0.03, 0.1, 0.3, 1, 3, and 10 mM.

To analyze for 2,3-dihydroxyisovalerate, the sample was diluted 10× withwater, and 8.0 μl was injected into a Waters Acquity HPLC equipped withWaters SQD mass spectrometer (Waters Corporation, Milford, Mass.). Thechromatography conditions were: flow rate (0.5 ml/min), on a WatersAcquity HSS T3 column (2.1 mm diameter, 100 mm length). Buffer Aconsisted of 0.1% (v/v) in water, Buffer B was 0.1% formic acid inacetonitrile. The sample was analyzed using 1% buffer B (in buffer A)for 1 min, followed by a linear gradient from 1% buffer B at 1 min to75% buffer B at 1.5 min. The reaction product, 2,3-dihydroxyisovalerate,was detected by ionization at m/z=133, using the electrospray ionizationdevise at −30 V cone voltage. The amount of product2,3-dihydroxyisovalerate was calculated by comparison to an authenticstandard.

To calculate the K_(M) for NADH, the rate data for DHIV formation wasplotted in Kaleidagraph (Synergy Software, Reading, Pa.) and fitted tothe single substrate Michaelis-Menton equation, assuming saturatingacetolactate concentration.

Example 1 Construction of Site-Saturation Gene Libraries to IdentifyMutants Accepting NADH as Cofactor

Seven gene libraries were constructed (Table 2) using two steps: 1)synthesis of Megaprimers using commercially synthesizedoligonucleotidies described in Table 1; and 2) construction of mutatedgenes using the Megaprimers obtained in step 1. These primers wereprepared using high fidelity pfu-ultra polymerase (Stratagene, La Jolla,Calif.) for one pair of primer containing one forward and one reverseprimer. The templates for libraries C, E, F, G and H were the wild typeof PF5_ilvc. The DNA templates for library N were those mutants havingdetectable NADH activity from library C while those for library O werethose mutants having detectable NADH activity from library H. A 50 μlreaction mixture contained: 5.01 of 10× reaction buffer supplied withthe pfu-ultra polymerase (Stratagene), 1.0 μl of 50 ng/μl template, 1.0μl each of 10 pmol/μl forward and reverse primers, 1.0 μl of 40 mM dNTPmix (Promega, Madison, Wis.), 1.0 μl pfu-ultra DNA polymerase(Stratagene) and 39 μl water. The mixture was placed in a thin well 200μl tube for the PCR reaction in a Mastercycler gradient equipment(Brinkmann Instruments, Inc. Westbury, N.Y.). The following conditionswere used for the PCR reaction: The starting temperature was 95° C. for30 sec followed by 30 heating/cooling cycles. Each cycle consisted of95° C. for 30 sec, 54° C. for 1 min, and 70° C. for 2 min. At thecompletion of the temperature cycling, the samples were kept at 70° C.for 4 min more, and then held awaiting sample recovery at 4° C. The PCRproduct was cleaned up using a DNA cleaning kit (Cat#D4003, ZymoResearch, Orange, Calif.) as recommended by the manufacturer.

TABLE 2 Gene Libraries Targeted Library position(s) name Templates ofPf5_ilvC Primers used C PF5_ilvc 47, 50, 52 and 53 SEQ ID No: 1 and 2 EPF5_ilvc 47 SEQ ID No: 1 and 3 F PF5_ilvc 50 SEQ ID No: 1 and 4 GPF5_ilvc 52 SEQ ID No: 1 and 5 H PF5_ilvc 47, 50, and 52 SEQ ID No: 1and 6 N Good mutants 53 SEQ ID NO: 20 and 21 from library C O Goodmutants 53 SEQ ID NO: 20 and 21 from library H

The Megaprimers were then used to generate gene libraries using theQuickChange II XL site directed mutagenesis kit (Catalog #200524,Stratagene, La Jolla Calif.). A 50 μl reaction mixture contained: 5.0 μlof 10× reaction buffer, 1.0 μl of 50 ng/μl template, 42 μl Megaprimer,1.0 μl of 40 mM dNTP mix, 1.0 μl pfu-ultra DNA polymerase. Except forthe Megaprimer and the templates, all reagents used here were suppliedwith the kit indicated above. This reaction mixture was placed in a thinwell 200 μl-capacity PCR tube and the following reactions were used forthe PCR: The starting temperature was 95° C. for 30 sec followed by 25heating/cooling cycles. Each cycle consisted of 95° C. for 30 sec, 55°C. for 1 min, and 68° C. for 6 min. At the completion of the temperaturecycling, the samples were kept at 68° C. for 8 min more, and then heldat 4° C. for later processing. Dpn I restriction enzyme (1.0 μl)(supplied with the kit above) was directly added to the finishedreaction mixture, enzyme digestion was performed at 37° C. for 1 h andthe PCR product was cleaned up using a DNA cleaning kit (Zymo Research).The cleaned PCR product (101) contained mutated genes for a genelibrary.

The cleaned PCR product was transformed into an electrocompetent strainof E. coli Bw25113 (ΔilvC) using a BioRad Gene Pulser II (Bio-RadLaboratories Inc., Hercules, Calif.). The transformed clones werestreaked on agar plates containing the LB medium and 100 μg/mlampicillin (Cat#L1004, Teknova Inc. Hollister, Calif.) and incubated at37° C. overnight. Dozens of clones were randomly chosen for DNAsequencing to confirm the quality of the library.

TABLE 3 List of some mutants having NADH activity identified fromsaturation libraries Mutant Position 47 Position 50 Position 52 Position53 SD2 R47Y S50A T52H V53W SB1 R47Y S50A T52G V53W SE1 R47A S50W T52GV53W SH2 R47N S50W T52N V53W SB2 R47I T52G V53W SG1 R47Y T52G V53W SB3R47G S50W T52G V53W SE2 R47P S50E T52A V53W SD3 R47L S50W T52G V53W C2A6R47I S50G T52D V53W C3E11 R47A S50M T52D V53W C3A7 R47Y S50A T52D V53WC3B11 R47F S50A T52D V53W C4A5 R47Y S50A T52S V53W C3B12 R47I T52D V53WC4H7 R47I T52S V53W C1D3 R47G S50M T52D V53W C4D12 R47C S50W T52G V53WC1G7 R47P S50G T52D V53W C2F6 R47P S50V T52D V53W C1C4 R47P S50E T52SV53W 6924F9 R47P S50G T52D 6881E11 R47P S50N T52C 6868F10 R47P T52S6883G10 R47P S50D T52S 6939G4 R47P S50C T52D 11463D8 R47P S50F T52D9667A11 R47N S50N T52D V53A 9675C8 R47Y S50A T52D V53A 9650E5 R47N S50WT52G V53H 9875B9 R47N S50N T52D V53W 9862B9 R47D S50W T52G V53W 9728G11R47N S50W T52G V53W 11461D8 R47F S50A T52D V53A 11461A2 R47P S50F T52DV53I

Example 2 Construction of Error Prone PCR Librar

Mutants obtained in Example 1, with mutations in their cofactor bindingsites which exhibited relatively good NADH activities, were used as theDNA template to prepare the error prone (ePCR) libraries using theGeneMorph II kit (Stratagene) as recommended by the manufacturer. Allthe epPCR libraries target the N-terminal (which contains the NADPHbinding site) of PF5_KARI. The forward primer (SED ID No: 20) and thereverse primer (SED ID No: 22) were used for all ePCR libraries.

The DNA templates for the n^(th) epPCR library were mutants having goodNADH activity from the (n−1)^(th) epPCR library. The templates of thefirst epPCR library were mutants having relatively good NADH activityfrom libraries N and O. The mutations rate of library made by this kitwas controlled by the amount of template added in the reaction mixtureand the number of amplification cycles. Typically, 1.0 ng of each DNAtemplate was used in 100 μL of reaction mixture. The number ofamplification cycles was 70. The following conditions were used for thePCR reaction: The starting temperature was 95° C. for 30 sec followed by70 heating/cooling cycles. Each cycle consisted of 95° C. for 30 sec,55° C. for 30 min, and 70° C. for 2 min. After the first 35heating/cooling cycles finished, more dNTP and Mutazyme II DNApolymerase were added. The PCR product was cleaned up using a DNAcleaning kit (Cat#D4003, Zymo Research, Orange, Calif.) as recommendedby the manufacturer. The cleaned PCR product was treated as Megaprimerand introduced into the vector using the Quickchange kit as described inExample 1. Table 4 below lists the KARI mutants obtained and thesignificant improvement observed in their NADH binding ability. TheK_(M) was reduced from 1100 μM for mutant C3B11 to 50 μM for mutant12957G9.

TABLE 4 List of some mutants with their measured K_(M) values NADHMutant Mutation Locations K_(M) (μM) C3B11 R47F/S50A/T52D/V53W 1100 SB3R47G/S50W/T52G/V53W 500 11518B4 R47N/S50N/T52D/V53A/A156V 141 11281G2R47N/S50N/T52D/V53A/A156V/L165M 130 12985F6R47Y/S50A/T52D/V53A/L61F/A156V 100 13002D8R47Y/S50A/T52D/V53A/L61F/A156V/G170A 68 12957G9Y24F/R47Y/S50A/T52D/V53A/L61F/G170A 50 12978D9R47Y/S50A/T52D/V53A/L61F/Q115L/A156V 114

Example 3 Identification of Amino Acids for Cofactor SpecificitySwitching Using Bioinformatic Tools

To discover if naturally existing KARI sequences could provide clues foramino acid positions that should be targeted for mutagenesis, multiplesequence alignment (MSA) using PF5_KARI, its close homolog PAO1_KARI andthree KARI sequences with measureable NADH activity, i.e., B. CereusilvC1 and ilvC2 and spinach KARI were performed (FIG. 8). Based on themultiple sequence alignment, positions 33, 43, 59, 61, 71, 80, 101, and119 were chosen for saturation mutagenesis. Saturation mutagenesis onall of these positions was performed simultaneously using theQuickChange II XL site directed mutagenesis kit (Catalog #200524,Stratagene, La Jolla Calif.) with the manufacturer's suggested protocol.Starting material for this mutagenesis was a mixed template consistingof the mutants already identified in Example 2, Table 4. The primersused are listed in Table 5. The library of mutants thus obtained werenamed “library Z”. Mutants with good NADH activity from this librarywere identified using high throughput screening and their KARI activityand the K_(M) for NADH were measured as described above. These mutants(Table 6) possess much lower K_(M) s for NADH compared to the parenttemplates (Table 4). A Megaprimer, using primers (SEQ ID Nos. 20 and58), was created and mutations at positions 156 and 170 were eliminated.Further screening of this set of mutants identified mutant 3361G8 (SEQID NO: 67) (Table 7). The hits from library Z were further subjected tosaturation mutagenesis at position 53 using primers (SEQ ID Nos. 20 and21), and subsequent screening identified the remaining mutants in Table7. As shown in Table 7 the new mutants possessed much lower K_(M) forNADH (e.g., 4.0 to 5.5 μM) compared to mutants listed in Table 6 (e.g.,14-40 μM).

TABLE 5 Primers for Example 5 Targeted position(s) of Pf5_ilvC Primers 33 pBAD-405-C33_090808f: GCTCAAGCANNKAACCTGAAGG (SEQ ID NO: 49)pBAD-427-C33_090808r: CCTTCAGGTTKNNTGCTTGAGC (SEQ ID NO: 50)  43pBAD-435-T43_090808f: GTAGACGTGNNKGTTGGCCTG (SEQ ID NO: 51)pBAD-456-T43_090808r: CAGGCCAACKNNCACGTCTAC (SEQ ID NO: 52) 59 and 61pBAD-484-H59L61_090808f: CTGAAGCCNNKGGCNNKAAAGTGAC (SEQ ID NO: 53)pBAD-509-H59L61_090808r: GTCACTTTKNNGCCKNNGGCTTCAG (SEQ ID NO: 54)  71pBAD-519-A71_090808f: GCAGCCGTTNNKGGTGCCGACT (SEQ ID NO: 55)pBAD-541-A71_090808r: AGTCGGCACCKNNAACGGCTGC (SEQ ID NO: 56)  80pBAD-545-T80_090808f: CATGATCCTGNNKCCGGACGAG (SEQ ID NO: 57)pBAD-567-T80_090808r: CTCGTCCGGKNNCAGGATCATG (SEQ ID NO: 58) 101pBAD-608-A101_090808f: CAAGAAGGGCNNKACTCTGGCCT (SEQ ID NO: 59)pBAD-631-A101_090808r: AGGCCAGAGTKNNGCCCTTCTTG (SEQ ID NO: 60) 119pBAD-663-R119_090808f: GTTGTGCCTNNKGCCGACCTCG (SEQ ID NO: 61)pBAD-685-R119_090808r: CGAGGTCGGCKNNAGGCACAAC (SEQ ID NO: 62)

TABLE 6 List of some mutants with their measured K_(M) values (positionsto be mutated in this library were indentified by bioinformatic tools)NADH Mutant Mutation Locations K_(M) (μM) ZB1Y24F/R47Y/S50A/T52D/V53A/L61F/A156V 40 (SEQ ID NO: 24) ZF3Y24F/C33L/R47Y/S50A/T52D/V53A/L61F 21 (SEQ ID NO: 25) ZF2Y24F/C33L/R47Y/S50A/T52D/V53A/L61F/A156V 17 (SEQ ID NO: 26) ZB3Y24F/C33L/R47Y/S50A/T52D/V53A/L61F/G170A 17 (SEQ ID NO: 27) Z4B8C33L/R47Y/S50A/T52D/V53A/L61F/T80I/A156V 14 (SEQ ID NO: 28)

TABLE 7 Mutants further optimized for improved K_(M) (for NADH) NADHMutant Mutation Locations K_(m) (μM) 3361G8C33L/R47Y/S50A/T52D/V53A/L61F/T80I 5.5 (SEQ ID NO: 67) 2H10Y24F/C33L/R47Y/S50A/T52D/V53I/L61F/T80I/ 5.3 A156V (SEQ ID NO: 68) 1D2Y24F/R47Y/S50A/T52D/V53A/L61F/T80I/ 4.1 A156V (SEQ ID NO: 69) 3F12Y24F/C33L/R47Y/S50A/T52D/V53A/L61F/T80I/ 4.0 A156V (SEQ ID NO: 70)3361E1 Y24F/R47Y/S50A/T52D/V53I/L61F 4.5 (SEQ ID NO: 84)

Further analyses using bioinformatic tools were therefore performed toexpand the mutational sites to other KARI sequences as described below.

Sequence Analysis

Members of the protein family of ketol-acid reducoisomorase (KARI) wereidentified through BlastP searches of publicly available databases usingamino acid sequence of Pseudomonas fluorescens PF5 KARI (SEQ ID NO:17)with the following search parameters: E value=10, word size=3,Matrix=Blosum62, and Gap opening=11 and gap extension=1, E value cutoffof 10⁻³. Identical sequences and sequences that were shorter than 260amino acids were removed. In addition, sequences that lack the typicalGxGXX(G/A) motif involved in the binding of NAD(P)H in the N-terminaldomain were also removed. These analyses resulted in a set of 692 KARIsequences.

A profile HMM was generated from the set of the experimentally verifiedClass I and Class II KARI enzymes from various sources as described inTable 8. Details on building, calibrating, and searching with thisprofile HMM are provided below. Any sequence that can be retrieved byHMM search using the profile HMM for KARI at E-value above 1E⁻³ isconsidered a member of the KARI family. Positions in a KARI sequencealigned to the following in the profile HMM nodes (defined below in thesection of profile HMM building) are claimed to be responsible for NADHutilization: 24, 33, 47, 50, 52, 53, 61, 80, 115, 156, and 170 (thenumbering is based on the sequences of Pseudomonas fluorescens PF5KARI).

Preparation of Profile HMM

A group of KARI sequences were expressed in E. coli and have beenverified to have KARI activity These KARIs are listed in Table 6. Theamino acid sequences of these experimentally verified functional KARIswere analyzed using the HMMER software package (The theory behindprofile HMMs is described in R. Durbin, S. Eddy, A. Krogh, and G.Mitchison, Biological sequence analysis: probabilistic models ofproteins and nucleic acids, Cambridge University Press, 1998; Krogh etal., J. Mol. Biol. 235:1501-1531, 1994), following the user guide whichis available from HMMER (Janelia Farm Research Campus, Ashburn, Va.).The output of the HMMER software program is a profile Hidden MarkovModel (profile HMM) that characterizes the input sequences. As stated inthe user guide, profile HMMs are statistical descriptions of theconsensus of a multiple sequence alignment. They use position-specificscores for amino acids (or nucleotides) and position specific scores foropening and extending an insertion or deletion. Compared to otherprofile based methods, HMMs have a formal probabilistic basis. ProfileHMMs for a large number of protein families are publicly available inthe PFAM database (Janelia Farm Research Campus, Ashbum, Va.).

The profile HMM was built as follows:

Step 1. Build a Sequence Alignment

The 25 sequences for the functionally verified KARIs listed above werealigned using Clustal W (Thompson, J. D., Higgins, D. G., and Gibson T.J., Nuc. Acid Res. 22: 4673 4680, 1994) with default parameters. Thealignment is shown in FIG. 9.

TABLE 8 25 Experimentally verified KARI enzymes GI SEQ Number AccessionID NO: Microorganism 70732562 YP_262325.1 17 Pseudomonas fluorescensPf-5 15897495 NP_342100.1 13 Sulfolobus solfataricus P2 18313972NP_560639.1 14 Pyrobaculum aerophilum str. IM2 76801743 YP_326751.1 30Natronomonas pharaonis DSM 2160 16079881 NP_390707.1 31 Bacillussubtilis subsp. subtilis str. 168 19552493 NP_600495.1 32Corynebacterium glutamicum ATCC 13032 6225553 O32414 33 Phaeospririlummolischianum 17546794 NP_520196.1 15 Ralstonia solanacearum GMI100056552037 YP_162876.1 34 Zymomonas mobilis subsp. mobilis ZM4 114319705YP_741388.1 35 Alkalilimnicola ehrlichei MLHE-1 57240359 ZP_00368308.136 Campylobacter lari RM2100 120553816 YP_958167.1 37 Marinobacteraquaeolei VT8 71065099 YP_263826.1 38 Psychrobacter arcticus 273-483648555 YP_436990.1 39 Hahella chejuensis KCTC 2396 74318007YP_315747.1 40 Thiobacillus denitrificans ATCC 25259 67159493ZP_00420011.1 41 Azotobacter vinelandii AvOP 66044103 YP_233944.1 42Pseudomonas syringae pv. syringae B728a 28868203 NP_790822.1 43Pseudomonas syringae pv. tomato str. DC3000 26991362 NP_746787.1 44Pseudomonas putida KT2440 104783656 YP_610154.1 45 Pseudomonasentomophila L48 146306044 YP_001186509.1 46 Pseudomonas mendocina ymp15599888 NP_253382.1 16 Pseudomonas aeruginosa PAO1 42780593 NP_977840.147 Bacillus cereus ATCC 10987 42781005 NP_978252.1 48 Bacillus cereusATCC 10987 266346 Q01292 18 Spinacia oleracea

Step 2. Build a Profile HMM

The hmmbuild program was run on the set of aligned sequences usingdefault parameters. hmmbuild reads the multiple sequence alignment file,builds a new profile HMM, and saves the profile HMM to file. Using thisprogram an un-calibrated profile was generated from the multiplesequence alignment for twenty-four experimentally verified KARIs asdescribed above.

The following information based on the HMMER software user guide givessome description of the way that the hmmbuild program prepares a profileHMM. A profile HMM is a linear state machine consisting of a series ofnodes, each of which corresponds roughly to a position (column) in themultiple sequence alignment from which it is built. If gaps are ignored,the correspondence is exact, i.e., the profile HMM has a node for eachcolumn in the alignment, and each node can exist in one state, a matchstate. The word “match” here implies that there is a position in themodel for every position in the sequence to be aligned to the model.Gaps are modeled using insertion (I) states and deletion (D) states. Allcolumns that contain more than a certain fraction x of gap characterswill be assigned as an insert column. By default, x is set to 0.5. Eachmatch state has an I and a D state associated with it. HMMER calls agroup of three states (M/D/I) at the same consensus position in thealignment a “node”.

A profile HMM has several types of probabilities associated with it. Onetype is the transition probability—the probability of transitioning fromone state to another. There are also emissions probabilities associatedwith each match state, based on the probability of a given residueexisting at that position in the alignment. For example, for a fairlywell-conserved column in an alignment, the emissions probability for themost common amino acid may be 0.81, while for each of the other 19 aminoacids it may be 0.01.

A profile HMM is completely described in a HMMER2 profile save file,which contains all the probabilities that are used to parameterize theHMM. The emission probabilities of a match state or an insert state arestored as log-odds ratio relative to a null model: log₂ (p_x)/(null_x).Where p_x is the probability of an amino acid residue, at a particularposition in the alignment, according to the profile HMM and null_x isthe probability according to the Null model. The Null model is a simpleone state probabilistic model with pre-calculated set of emissionprobabilities for each of the 20 amino acids derived from thedistribution of amino acids in the SWISSPROT release 24. Statetransition scores are also stored as log odds parameters and areproportional to log₂(t_x). Where t_x is the transition probability oftransiting from one state to another state.

Step 3. Calibrate the Profile HMM

The profile HMM was read using hmmcalibrate which scores a large numberof synthesized random sequences with the profile (the default number ofsynthetic sequences used is 5,000), fits an extreme value distribution(EVD) to the histogram of those scores, and re-saves the HMM file nowincluding the EVD parameters. These EVD parameters (μ and λ) are used tocalculate the E-values of bit scores when the profile is searchedagainst a protein sequence database. Hmmcalibrate writes two parametersinto the HMM file on a line labeled “EVD”: these parameters are the μ(location) and λ(scale) parameters of an extreme value distribution(EVD) that best fits a histogram of scores calculated on randomlygenerated sequences of about the same length and residue composition asSWISS-PROT. This calibration was done once for the profile HMM.

The calibrated profile HMM for the set of KARI sequences is providedappended hereto as a profile HMM Excel chart (Table 9). In the mainmodel section starting from the HMM flag line, the model has three linesper node, for M nodes (where M is the number of match states, as givenby the LENG line). The first line reports the match emission log-oddsscores: the log-odds ratio of emitting each amino acid from that stateand from the Null model. The first number if the node number (1 . . .M). The next K numbers for match emission scores, one per amino acid.The highest scoring amino acid is indicated in the parenthesis after thenode number. These log-odds scores can be converted back to HMMprobabilities using the null model probability. The last number on theline represents the alignment column index for this match state. Thesecond line reports the insert emission scores, and the third linereports on state transition scores: M→M, M→I, M→D; I—M, I—I; D→M, D→D;B→M; M→E.

Step 4. Test the Specificity and Sensitivity of the Built Profile HMMs

The Profile HMM was evaluated using hmmsearch, which reads a Profile HMMfrom hmmfile and searches a sequence file for significantly similarsequence matches. The sequence file searched contained 692 sequences(see above). During the search, the size of the database (Z parameter)was set to 1 billion. This size setting ensures that significantE-values against the current database will remain significant in theforeseeable future. The E-value cutoff was set at 10.

An hmmersearch, using hmmsearch, with the profile HMM generated from thealignment of the twenty-five KARIs with experimentally verifiedfunction, matched all 692 sequences with an E value<10⁻³. This resultindicates that members of the KARI family share significant sequencesimilarity. A hmmersearch with a cutoff of E value 10⁻³ was used toseparate KARIs from other proteins.

Step 5. Identify Positions that are Relevant for NAD(P)H Utilization.

Eleven positions have been identified in KARI of Pseudomonas fluorescensPf-5 that switches the cofactor from NADPH to NADH. Since the KARIsequences share significant sequence similarity (as described above), itcan be reasoned that the homologous positions in the alignment of KARIsequences should contribute to the same functional specificity. Theprofile HMM for KARI enzymes has been generated from the multiplesequence alignment which contains the sequence of Pseudomonasfluorescens Pf-5 KARI. The eleven positions in the profile HMMrepresenting the columns in the alignment which correspond to the elevencofactor switching positions in Pseudomonas fluorescens Pf-5 KARI areidentified as positions 24, 33, 47, 50, 52, 53, 61, 80, 115, 156, and170. The lines corresponding to these positions in the model file arehighlighted in yellow in Table 9.

For any query sequence, hmm search is used to search the profile HMM forKARI against the query sequence and the alignment of the query to theHMM is recorded in the output file. In the alignment section of theoutput, the top line is the HMM consensus. The amino acid shown for theconsensus is the highest probability amino acid at that positionaccording to the HMM (not necessarily the highest scoring amino acid).The center line shows letters for “exact” matches to the highestprobability residue in the HMM, or a “+” when the match has a positivescore. The third line shows the sequence itself. The positions in thequery sequence that are deemed as relevant for cofactor switching areidentified as those that are aligned to these eleven nodes in theprofile HMM as described above. An example of the alignment ofPseudomonas fluorescens Pf-5 KARI to the profile HMM of KARI is shown inFIG. 10 and the eleven positions that are responsible for cofactorswitching are shaded in grey.

Example 4 Construction of a Site-Saturation Gene Library for CompleteCofactor Switching to Nadh

To construct the site-saturation gene library for KARI mutants, mutants3361E1, 3361G8, 1D2, 2H10, 3F12, & Z4B8 (see Example 3, Tables 6 and 7)were used as templates. The library was constructed using QuickChangekit (Cat#200524, Stratagene, La Jolla, Calif.). The concentration ofeach mutant in the template mixture was 5.0 ng/μl. The two primers (2.5nM) introducing saturation mutagenesis at positions 47, 50, 52 and 53,were PF5_(—)4Mt111008.f (SEQ ID NO: 71) and PF5_(—)4Mt111008.r (SEQ IDNO: 72).

The PCR Reaction Mixture Contained:

10 × reaction buffer 5.0 μl PF5_4Mt111008.f 2.0 μl PF5_4Mt111008.r 2.0μl 50 × dNTP 1.0 μl DNA Template 1.0 μl PfuUltra 1.0 μl Water  38 μl

The PCR Reaction Program was:

1) 95° C. 30 sec 2) 95° C. 30 sec 3) 55° C. 1.0 min 4) 68° C. 6.0 min 5)Go to step (2) Repeat 35 times 6) 68° C. 8.0 min 7) 4° C. press Enter

The mixture was placed in a thin well 200 μl tube for the PCR reactionin a Mastercycler gradient equipment (Brinkmann Instruments, Inc.Westbury, N.Y.). After the PCR reaction, 1.0 μl Dpn I restriction enzyme(supplied with the kit above) was directly added into the PCR reactionmixture, which was then incubated at 37° C. for 1 h to remove the DNAtemplates. The Dpn I digested PCR product was cleaned up by the Zymo DNAclearance kit (Cat#D4003, Zymo Research, Orange, Calif.) as recommendedby the manufacturer.

The cleaned PCR product was transformed into an electro-competent strainof E. coli Bw25113 (LilvC) using a BioRad Gene Pulser II (Bio-RadLaboratories Inc., Hercules, Calif.). The transformed clones werestreaked on agar plates containing the LB medium and 100 μg/mlampicillin (Cat#L1004, Teknova Inc. Hollister, Calif.) and incubated at37° C. overnight. Dozens of clones were randomly chosen for DNAsequencing to confirm the quality of the library. Several mutantsidentified in this library (Table 10 and FIG. 11) had very low NADPHactivity while they had good NADH activity. Their cofactor consumptionis listed in Table 11 (The data was based on three parallelmeasurements). “Negative” in the following Tables refers to an emptypBAD vector without the KARI gene.

TABLE 10 List of some of the mutants identified in Example 1 MutantMutation Locations JB1C6 Y24F/C33L/R47H/S50D/T52Y/V53Y/L61F/T80I/A156V16445E4 C33L/R47P/S50V/T52D/V53G/L61F/T80I/A156V 16468D7Y24F/C33L/R47T/S50I/T52D/V53R/L61F/T80I/A156V 16469F3C33L/R47E/S50A/T52D/V53A/L61F/T80I

TABLE 11 The cofactor consumption of some mutants following a 5 minreaction (decrease in OD₃₄₀ nm) 0.2 mM NADH 0.2 mM NADPH Mutants averagestdev average stdev JB1C6 −0.232 0.127 −0.019 0.009 16445E4 −0.152 0.057−0.013 0.001 16468D7 −0.153 0.012 −0.039 0.020 16469F3 −0.054 0.069−0.025 0.016 Z4B8 −0.178 0.042 −0.170 0.013 PF5_WT −0.078 0.014 −0.3200.024 Negative −0.061 0.029 −0.015 0.014

Example 5 Construction of a Domain Swapping Library

In this Example the beneficial mutations outside the cofactor bindingsites and the beneficial mutations within the cofactor binding siteswere combined to create a domain swapping library.

Mutants, which had mutations in the cofactor binding site and exhibitedonly NADH activity (SE1, SB3, SE2, SD3, C2F6, C3B11, C4D12, 9650E5,9667A11, 9862B9, 9875B9, 11461D8, 11463D8, 11518B4, SEQ ID NOs: 85-98),were used to obtain additional beneficial mutations in the cofactorbinding site. Two primers, pBAD_(—)230f (SEQ ID NO: 73) andpBAD_(—)601_(—)021308r (SEQ ID NO: 74), were used to amplify the mutantslisted in Table 12. PCR reagents used were from Invitrogen(Cat#10572-014, Invitrogen, Carlsbad, Calif.).

The PR Reaction Mixture Contained:

PCR SuperMix 180 μl  pBAD_230.f (18 nM) 5.0 μl pBAD_601_021308r (10 nM)9.0 μl Template mix (5.0 ng/μl) 6.0 μl

The PCR Reaction Program was:

(1) 95° C. 30 sec (2) 95° C. 20 sec (3) 55° C. 20 sec (4) 72° C. 60 sec(5) Go to step (2) repeat 35 times (6) 72° C. 4 min (7) 4° C. pressenter

After the PCR reaction, 1.0 μl Dpn I restriction enzyme (supplied withthe kit above) was directly added into the PCR reaction mixture, whichwas then incubated at 37° C. for 1 h to remove the DNA templates. TheDpn I digested PCR product was cleaned up by the Zymo DNA clearance kit(Cat#D4003, Zymo Research, Orange, Calif.) as recommended by themanufacturer and 42 μl cleaned DNA product containing beneficialmutations in the cofactor binding sites obtained was designated asMegaprimer.

The Megaprimers thus obtained were then used to generate the domainswapping library using the QuickChange II XL site directed mutagenesiskit (Catalog #200524, Stratagene, La Jolla Calif.). The templates usedin Example 4 were also used in this experiment. A 50 μl reaction mixturecontaining: 5.0 μl of 10× reaction buffer, 1.01 of 5.0 ng/μl template,42 μl Megaprimer, 1.0 μl of 40 mM dNTP mix, 1.0 μl pfu-ultra DNApolymerase was prepared. Except for the Megaprimer and the templates,all reagents used here were supplied with the purchased kit. Thisreaction mixture was placed in a thin well 200 μl capacity PCR tube andthe following reactions were used for the PCR. The starting temperaturewas 95° C. for 30 sec followed by 30 heating/cooling cycles. Each cycleconsisted of 95° C. for 30 sec, 55° C. for 1 min, and 68° C. for 6 min.At the completion of the temperature cycling, the samples were kept at68° C. for 8 min, and then stored at 4° C. for later processing. Dpn Irestriction enzyme (1.0 μl) (supplied with the kit above) was directlyadded to the finished reaction mixture, enzyme digestion was performedat 37° C. for 1 h and the PCR product was cleaned up using a DNAcleaning kit (Zymo Research). The cleaned PCR product (10 μl) containedmutated genes for a gene library.

The mutated genes were transformed into an electro-competent strain ofE. coli Bw25113 (ΔilvC) using a BioRad Gene Pulser II (Bio-RadLaboratories Inc., Hercules, Calif.). The transformed clones werestreaked on LB agar plates containing 100 μg/ml ampicillin (Cat#L1004,Teknova Inc. Hollister, Calif.) and incubated at 37° C. overnight.Dozens of clones were randomly chosen for DNA sequencing to confirm thequality of the library.

This library yielded many mutants with high NADH activity (low K_(M) forNADH), which also had very low NADPH activity. (Table 12 and FIG. 12).Their cofactor consumption is also shown in Table 13 (The data was basedon three parallel measurements).

TABLE 12 Mutants with improved K_(M) (for NADH) obtained from the domainswapping library NADH Mu- K_(M) tant Mutation Locations (μM) JEA1Y24F/C33L/R47P/S50F/T52D/L61F/T80I/A156V 9.1 JEG2Y24F/C33L/R47F/S50A/T52D/V53A/L61F/T80I/A156V 9.4 JEG4Y24F/C33L/R47N/S50N/T52D/V53A/L61F/T80I/A156V 9.6 JEA7Y24F/C33L/R47P/S50N/T52D/V53A/L61F/T80I/A156V 10.6 JED1C33L/R47N/S50N/T52D/V53A/L61F/T80I/A156V 11.0

TABLE 13 The cofactor consumption of some mutants after 5 min reaction(decrease in OD₃₄₀ nm) 0.2 mM NADH 0.2 mM NADPH Mutants average stdevaverage stdev JEA1 −0.285 0.030 −0.110 0.025 JED1 −0.287 0.032 −0.0740.014 JEG2 −0.261 0.009 −0.078 0.009 JEG4 −0.227 0.016 −0.050 0.016 JEA7−0.205 0.079 −0.038 0.009 Z4B8 −0.178 0.042 −0.170 0.013 PF5_WT −0.0780.014 −0.320 0.024 Negative −0.061 0.029 −0.015 0.014

Example 6 Thermostability of Pf5-Ilvc and its Mutants

The wildtype PF5-ILVC and various cells containing mutated pBad-ilvCwere grown overnight at 37° C. in 25 ml of the LB medium containing 100μg/ml ampicillin and 0.02% (w/v) arabinose inducer while shaking at 250rpm. The cells were then harvested by centrifugation at 18,000×g for 1min at room temperature and the cell pellets were re-suspended in 300 μlof BugBuster Master Mix (EMD Chemicals). The reaction mixture was firstincubated at room temperature for 20 min and aliquots of this cellmixture (e.g. 50 μl) were incubated at different temperatures (from roomtemperature to 75° C.) for 10 min. The precipitate was removed bycentrifugation at 18,000×g for 5 min at room temperature. The remainingactivity of the supernatant was analyzed as described above. As shown inFIG. 7, pBad-ilvC was very stable with T₅₀ at 68° C. (T₅₀ is thetemperature, at which 50% of protein lost its activity after 10 minincubation).

The thermostability of PF5-ilvC allowed destruction of most of the othernon-KARI NADH oxidation activity within these cells, reducing the NADHbackground consumption and thus facilitating the KARI activity assays.This heat treatment protocol was used in all screening and re-screeningassays. The mutants thus obtained were all thermostable which allowedeasier selection of the desirable mutants.

Example 7 Stoichiometric Production of 2,3-Dihydroxyisovalerate By KariDuring Consumption of NADH or NADPH as Cofactors

Screening and routine assays of KARI activity rely on the 340 nmabsorption decrease associated with oxidation of the pyridinenucleotides NADPH or NADH. To insure that this metric was coupled to theformation of the reaction product (i.e., 2,3-dihydroxyisovalerate),oxidation of both pyridine nucleotide and formation of2,3-dihydroxyisovalerate were measured in the same samples.

The oxidation of NADH or NADPH was measured at 340 nm in a 1 cm pathlength cuvette on a Agilent model 8453 spectrophotometer (AgilentTechnologies, Wilmington Del.). Crude cell extract (0.1 ml) prepared asdescribed above containing either wild type PF5 KARI or the C3B11mutant, was added to 0.9 ml of K-phosphate buffer (10 mM, pH 7.6),containing 10 mM MgCl₂, and 0.2 mM of either NADPH or NADH. The reactionwas initiated by the addition of acetolactate to a final concentrationof 0.4 mM. After 10-20% decrease in the absorption (about 5 min), 50 μlof the reaction mixture was rapidly withdrawn and added to a 1.5 mlEppendorf tube containing 10 μl 0.5 mM EDTA to stop the reaction and theactual absorption decrease for each sample was accurately recorded.Production of 2,3-dihydroxyisovalerate was measured and quantitated byHPLC/MS as described above.

The coupling ratio is defined by the ratio between the amount of2,3-dihydroxyisovalerate (DHIV) produced and the amount of either NADHor NADPH consumed during the experiment. The coupling ratio for the wildtype enzyme (PF5-ilvC), using NADPH, was 0.98 DHIV/NADPH, while that forthe mutant (C3B11), using NADH, was on average around 1.10 DHIVINADPHunderlining the high activity of the mutant enzyme to consume NADH andproduce DHIV.

1-18. (canceled)
 19. A recombinant host cell comprising a mutantketol-acid reductoisomerase enzyme, wherein said enzyme has a ketol-acidreductoisomerase HMM search profile E value of <10⁻³ using the hmmsearchprogram and comprises at least one mutation within the phosphate bindingloop of said enzyme wherein said at least one mutation comprises asubstitution at the residue corresponding to T52 of SEQ ID NO: 17wherein the substitution at residue T52 is a substitution to a polar,negatively charged amino acid.
 20. The recombinant host cell of claim19, wherein the recombinant host cell is a member of a genera selectedfrom Clostridium, Zymomonas, Escherichia, Salmonella, Rhodococcus,Pseudomonas, Bacillus, Vibrio, Lactobacillus, Enterococcus, Alcaligenes,Klebsiella, Paenibacillus, Arthobacter, Corynebacterium, Brevibacterium,Pichia, Candida, Hansenula, and Saccharomyces.
 21. The recombinant hostcell of claim 20, wherein the host cell is a Saccharomyces.
 22. Therecombinant host cell of claim 19, wherein the mutant ketol-acidreductoisomerase enzyme comprises a GxGxx(G/A) cofactor binding motif.23. The recombinant host cell of claim 19, wherein the host cell furthercomprises an acetolactate synthase enzyme.
 24. The recombinant host cellof claim 23, wherein the acetolactate synthase enzyme is a Bacillussubtilis acetolactate synthase enzyme.
 25. A method of convertingacetolactate to 2,3-dihydroxyisovalerate, the method comprising: (a)providing a recombinant host cell comprising a mutant ketol-acidreductoisomerase enzyme, wherein said enzyme has a ketol-acidreductoisomerase HMM search profile E value of <10⁻³ using the hmmsearchprogram and comprises at least one mutation within the phosphate bindingloop of said enzyme wherein said at least one mutation comprises asubstitution at the residue corresponding to T52 of SEQ ID NO: 17wherein the substitution at residue T52 is a substitution to a polar,negatively charged amino acid; and (b) contacting the recombinant hostcell with acetolactate, wherein the recombinant host cell convertsacetolactate to 2,3-dihydroxyisovalerate.
 26. The method of claim 25,wherein the recombinant host cell is a member of a genera selected fromClostridium, Zymomonas, Escherichia, Salmonella, Rhodococcus,Pseudomonas, Bacillus, Vibrio, Lactobacillus, Enterococcus, Alcaligenes,Klebsiella, Paenibacillus, Arthobacter, Corynebacterium, Brevibacterium,Pichia, Candida, Hansenula, and Saccharomyces.
 27. The method of claim26, wherein the recombinant host cell is a Saccharomyces.
 28. The methodof claim 25, wherein the mutant ketol-acid reductoisomerase enzymecomprises a GxGxx(G/A) cofactor binding motif.
 29. A method of producingisobutanol, the method comprising: (a) providing a recombinant host cellcomprising an isobutanol biosynthetic pathway, wherein the isobutanolbiosynthetic pathway comprises a mutant ketol-acid reductoisomeraseenzyme, wherein the enzyme has a ketol-acid reductoisomerase HMM searchprofile E value of <10⁻³ using the hmmsearch program and comprises atleast one mutation within the phosphate binding loop of said enzymewherein said at least one mutation comprises a substitution at theresidue corresponding to T52 of SEQ ID NO: 17 wherein the substitutionat residue T52 is a substitution to a polar, negatively charged aminoacid; and (b) contacting the recombinant host cell with a fermentablecarbon substrate, wherein the recombinant host cell produces isobutanol.30. The method of claim 29, wherein the recombinant host cell is amember of a genera selected from Clostridium, Zymomonas, Escherichia,Salmonella, Rhodococcus, Pseudomonas, Bacillus, Vibrio, Lactobacillus,Enterococcus, Alcaligenes, Klebsiella, Paenibacillus, Arthobacter,Corynebacterium, Brevibacterium, Pichia, Candida, Hansenula, andSaccharomyces.
 31. The method of claim 30, wherein the recombinant hostcell is a Saccharomyces.
 32. The method of claim 29, wherein the mutantketol-acid reductoisomerase enzyme comprises a GxGxx(G/A) cofactorbinding motif.
 33. The method of claim 29, wherein the recombinant hostcell further comprises an acetolactate synthase enzyme.
 34. The methodof claim 33, wherein the acetolactate synthase enzyme is a Bacillussubtilus acetolactate synthase enzyme.